If they’d make this the default a lot of leas tech-savvy people would regularly lose their data because regular account recovery mechanisms don’t work with E2EE enabled. The vast majority of people don’t even use password managers and yes, people forget their passwords and yes, the same thing happens with a 28-digit recovery phrase. No, many won’t remember where they put it when they wrote it down. Many won’t even understand what this phrase means, even when the setup process directly explains it to them.
But we can obviously also be all negative about why this isn’t enabled by default and make assumptions.