> It doesn’t seem very common for sales to happen without the founder sticking around though.

Typically there's a reason to keep founders around and it's not related to their output, but more as a figure head/tacit acceptance of the deal that is made.

If a larger company is swallowing a smaller company, they may want to continue to work with the founder/some members of the exec team to help with integration efforts for the companies.

In return, the members of this team typically added incentives and ability to generate even more wealth as a result.