A few comments about this.
A very small fraction of logins get the CAPTCHA challenge. We, and other services, face unrelenting brute force attacks on our login endpoints. If you are seeing a CAPTCHA on login, chances are that something about your connection is suspicious to our system. It's far from perfect, and we continue to improve it, but at most a percent or two of users are seeing CAPTCHA at any time.
The CAPTCHA is run in an iframe on a separate domain to sandbox it from the Proton login flow prevent it from compromising the webapp. Obviously Google still gets some information, but we do all we can to limit this.
CAPTCHAs are very hard to build, especially considering Google has a habit of clearing the field with it's own captcha-breaking code. Most companies do not have the resources to build their own. We had an alternative CAPTCHA we were going to use as a replacement a few years ago and then the company behind it went bankrupt. We are currently looking to replace ReCAPTCHA with hcaptcha, which should alleviate some of these problems.
We have other strategies which we are also exploring to try to reduce the need for CAPTCHAs entirely, but these are also not trivial to build and integrate into all clients.
TL;DR It's a small fraction of users who are affected, it's necessary to protect our users from brute force login attacks, we don't like it either and are working hard on replacements.