Other people will articulate better reasons than I will.

Anything that saves you password obscured can be unobscured on a local computer.

One method: browser dev tools. Change type="password" to type="text" and viola password unobscured.

I'm sure there's lots of other reasons.

I'm not saying don't do the best you can with the tools available, just warning against overconfidence.

it depends on which we’re talking about, but even proton’s has its flaws.