Why almost all free HTTP Proxy server doesn’t change the User-Agent?

I’ve created a automated test suite for proxy server. From several websites in the internet like https://free-proxy-list.net/ I get many IPs of free proxy servers. I’ve also created a own Website with JSON API to get HTTP requests header.

All requests mostly differ in values like X-Forwarded-For, Via and X-Proxy-Id. But no proxy server changes the User-Agent. Its always the User-Agent from the origin, in my case python-requests/2.20.0.

After researching I know it’s technical no problem to do this with a proxy but anyone now why no free http proxy server do it?