Projects like IPFS aim to revolutionize the Internet itself by laying it on new protocols, making it more stable and persistent; at the cost of the need of users seeding content, though.
In a torrent-like system, it's frequently an issue of having someone keeping files on their computer to make it available for others. Without this principle, similar peer-to-peer system can't exist.
It assumes and demeands responsible behavior: something not everyone has. Several users would wipe out every data downloaded to free disk space.
In such a system, how can even only one website encourage (or in worst case scenario: enforce) its userbase to seed the contents of the site itself, at the very least the part they already visited?
Without it, projects like IPFS cannot work on the grand scale, failing their vision of becoming a viable alternative for the masses.