Yes you can keep a hatchling in its adult enclosure, but its going to require more from you. Here's how.

Do I have to use a baby tank?

Truth is, it is easier to set up a hatchling in a smaller tank. There are benefits to it which is why people recommend it-but No, it isn't mandatory. They don't have baby tanks in the wild. You can make that adult sized enclosure work it just requires more of you. 

So the break down is this:

They need to feel safe. 
 
If they don't feel safe to travel to their water or various microclimates, they might not do it, so they won't thermoregulate, and they'll get sick. 
If they don't feel safe, they won't eat
If they don't feel safe, they will run and may try to bite.

It's a big deal for them, and for your experience with them.

When you're so small, it's easier to feel safe when you have a smaller space to worry about a predator being in than a larger one.  In a larger space there's more space for well... Open space.. where they think they could be spotted and ambushed by a predator in their goofy little minds. Being out in the open can feel very unsafe to a snake, especially if there isn't a place to hide immediately nearby if they needed to flee.

Security is simply a higher priority to a snake than enclosure size. This is why people swear their snake started doing better after returning them to a smaller space after a brief time in a larger one. They returned to a more familiar and therefore more secure environment = improvement. Its also why people report some snakes dont do as well in larger tanks. They often are designed by well meaning folks who want to give them as much open space as possible. We humans would appreciate that, but snakes are not like people in this way. Some snakes are simply terrified of wide open spaces.

but there does not have to be lots of open space in your large enclosure... 

That's the part people miss. You can make an enclosure that is both large and secure. 

I'm not saying its for everyone. It will be harder to clean. It will be harder to find them to keep an eye on them. It will cost more to heat. It will require more effort from you both in set up and maintenance. A lot of people prefer to keep them smaller at first, myself included. 

But yes it can be done.

Here's some tips to give you the best possible chance of success. 

What you must do is to make that larger space feel as safe to the snake as possible, and then give them the best start possible in the new environment.

You'll need to be extra sure that the enclosure is escape proof as hatchlings are quite good at exploiting holes. larger enclosures tend to have looser gap tolerances so this becomes an even bigger concern.  Silicone works great as crack filler in glass terrarium type tanks, just be sure to give it time to gas off. Safety first ya know?

I recommend blacking out 3 sides of the enclosure to help them feel more safe. Doesn't need to be fancy just some craft paper or anything that prevents them from seeing out of the glass.

You will need to provide at least two very tight fitting hides. Definitely read this article to get an idea on what's going to be required for hides. Place one on the cooler side and one on the warmer side.  Provide lots of clutter to hide within between the two since they'll be traveling a distance. You want them to be able to travel between them without being spotted by a predator en route.  Of course, we know there's no predators trying to eat them, but they don't. Not at first. 

Of course if you do have any predators (like cats/dogs/etc) that can see into the enclosure keep them away at first.

You are, in fact, a giant predator to them. Imagine a 200 foot alien that can rip the roof off your house, pick you up off the ground and speak some weird alien language at you. You'd be terrified. You may try to run or defend yourself, wouldn't you? Snakes would too, and that's what you are to them at first, a giant, ridiculously overpowered alien. So if they bite or flee it doesn't mean that they are "mean". It means they're scared, or you might be hurting them accidentally. 

Now odds are they've already been handled by a different human (its original breeder) and that's going to help, but keep in mind- they don't know you yet. So it's possible they will be scared of you at first. Just give them time to realize nothing is trying to eat them. 😂 

Since you're a predator too, you're going to want to avoid handling them for the first couple weeks, at least until they're eating for you. Eating is a very vulnerable thing. It makes them unable to quickly flee! So it helps a lot for them to feel safe to remove you from the equation- just for now.

To feed at first try to simply place the prey in front of the hide they spend their time in at night and back away. Turn the lights off if you can. you want them to think like they just stumbled across this nice fresh rodent. This is called drop feeding. It's the least intrusive method and I always recommend it during this more delicate acclimation time. They'll probably drag it into the hide to eat it at first. They can be shy eaters. That's normal.

Once a snake is eating under your care that's a good sign they are beginning to feel safe. That's when you can start building a relationship with them through handling. 

In time, your snake will see his home as a safe predator free space, and you as the bringer of all things good: food, water, cleaning, new experiences. It isnt automatic though, and it does typically take a little longer in a larger space simply due to the physics of being a larger area to get used to. That doesn't mean it shouldn't be done by people who are committed to the idea of providing what's needed for success. 

If you can give them a security oriented enclosure design with the proper temperature/humidity/prey for your species plus a little breathing room at move in, you'll be on your way to a long happy relationship with your new snake  🙂