First, as already mentioned, it wouldn't be same account.
Valve (Steam) isn't Google (Android) and/or Apple (iOS). It's three companies, three different entities. And each wants its piece of cake (or, simply put, your money). It is not developers decision, it is how it is.
But, you can install emulator on your PC and use same Android account, the game will work within emulator.
As already mentioned, real PC version sold on Steam is better, much more suited for playing on PC (mouse plus keyboard, wider viewing area, etc).
So, it is up to you to decide if it is worth paying again for different platform. But, you can play at home on your Android device(s), e.g. you can play on tablet, phone, you only need to use same account you purchased the game. Same goes for Steam version, you can play it on more than one PC, just use same Steam account o different PCs.
Save files are transferable.
EDIT: And yes, not to forget, on Steam there's often sale, just put it on wishlist and wait for next sale.