> Even when the outside interests don't write the legislation, it's the staff and not actually politicians that too: politicians are basically constantly in meetings / at events and have no time for such work.
That's a little bit true but mostly misleading. As with any office with professional staff, most of the work of a legislative office is done by the staff, not the head of the office, true. OTOH, that doesn't mean the the skill, knowledge, and understanding of the head of the office is irrelevant, as the head directs the staff strategically, reviews, inquires, refines, and redirects work, etc.
> Nothing prevents the staff from being retained from when legislator to the next.
The new legislators not having the skill to select appropriate staff might, but a bigger problem isn't that the staff aren't retained but that they usually are; the less skilled the legislators are, the more the professional staff (who probably are shuttling back and forth between working for legislators and working for interest groups) do not only the detail work, but also control the agenda, and the less which faces the electorate sends to fill the nominally most important seats actually matter.