I'm pretty sure it's just space consideration.
Filters are usually not a prevalent part of the form, so they shouldn't take too much space, so trying to optimize the space, they usually go for the more compact solution.
Now putting labels inside the fields, I think, has been popularized by the rise of Material Design system. They do use both ways now, tho - showing the label inside the field, but once you enter a value - above the field without changing the size of the field:
Note that, for example, Carbon Design System advises against using placeholders as labels:
Optional placeholder text provides hints or examples of what to enter. Placeholder text disappears after the user begins entering data into the input. As such, it should not be used as a replacement for a persistent label nor should it contain crucial information.