They generally require to have data visible on the server and/or handle independently encryption/decryption with related tools and key management (including key discovery).
For some, it might be worth, for 99% of the population who wouldn't be able to do this but also doesn't want their content availablento the provider, it's not.
There are certain things that are known facts, there is no need to prove them every time.
The simple fact that:
You can also see:
A recent talk, I will quote the preamble:
And you can find as many opinion pieces as you want, by just searching (for example: https://nullprogram.com/blog/2017/03/12/).
However, if you really believe I am wrong, and you disagree that PGP tooling is widely considered bad, complex and almost a meme in the security community, you are welcome to show where I am wrong. Show me a simple PGP setup that non-technical people use.
P.s.
I also found https://arxiv.org/pdf/1510.08555.pdf, an interesting paper which is a followup of another paper 10 years older about usability of PGP tools.