Because this topic makes people mad and makes politicians who oppose it look bad, which hurts electability. Not to mention it seems like non-elite politicians wouldn’t be affected, so for them this is an easy publicity boost.

In a decent democracy you could vote those out, sadly USA ain't that.

That's why populist movements raise its head from time to time. It's the human's self-cleansing mechanism -- very damaging, and usually burn much more than necessary, but whatever.

More importantly, why would you want to restrict public servants only to low interest rate government bonds? That would select out most people with decent financial sense from taking the job.

The point is to level the playing field, not to become so restrictive and punishing that the job only appeals to politicians who are so self-sacrificing that they don’t exist as much as people want. Stocks also aren’t the only place where politicians could leverage their positions for personal financial gain.

Unintended side benefit -- those with power are incentivized to raise bond interest, creating a safe investment for others to use.