Developers can - and should - lift systems, mechanics, and whatever the hell else they want from any other title (occasional sticky legal protections aside). The problem arises when those things are copied uncritically.
Good games are intricate systems of interlocking mechanics, and good game designers put a lot of work into designing those things to create the specific play experience they have in mind.
If you want to reuse some or all of those aspects, you need to also understand how and why they work, so that you can apply them correctly in a new context. If you don’t, and instead just lazily copy with no plan or vision? That’s when people start calling things out as rip-offs.