No, it isn't mandatory to use IPFS on ERC721 - as you already know, other than the token URI, typically, very little data about the asset is stored in the token on the blockchain, and the actual content linked to the token (i.e. MetaData and Image) can be served from anywhere on the internet.
However, IPFS is often used on NFTs to further the spirit of immutability / good faith. IPFS' content addressing scheme prevents changes to the data (since an attempt to update the image or metadata json would result in a new, different address).
Without a tamperproofing mechanism like IPFS CID, it is trivial for an NFT minter to change the metadata json or image content after the token has been minted or transferred onwards.