It’s not that you can’t at all, it’s just that you’d either need to give up a lot of the functionality of a lot of sites or at least reduce the usability of many sites and your browser or configure whitelist and such for every site manually and deal with breaking changes when websites update.
I have a separate boot partition so the rest can be encrypted with luks. That’s all that’s needed in a large majority of scenarios. Most other setups end up needing to resize something at some point which in many cases is a total pain.
On my primary PC I do have a second hard drive for documents and other long term storage files that I want to access more often than on the NAS. This way it’s nearly impossible to lose those files of I reinstall something and it can act as a temporary backup storage for settings files when I do reinstall stuff rather than having a partition that wastes space or runs out of space.