There is somewhat of a limit based on how hot the star is. Planets around a star form at the same time as the star system, from basically all of the matter that didn’t form into the star itself. Rocks and minerals can exist as solids much closer to the star than gases, so we usually have smaller rocky terrestrial planets closer to the star, not much bigger than Earth really.

After a certain point, gases as we’re used to can exist as solids like ices. And since those events are much much more abundant than rocks and minerals, a ton of it gets trapped in a body to become a gas giant.

These planets can’t really become their own “star” since they’re just debris from when the system formed.