“You only need a hechsher if you want to cater to a market that hold’s of the the stringent shita of that Rav.”
There’s your answer
“Ask yourself a simple question: If you met the Shaagas Aryeh, or the Radvaz, etc and told them that in 2022 we have a symbol that says water is kosher, what would they say to you?“
The same can be applied to fruits and vegetables but I can tell you for certain that there are numerous bugs in them