They don't have a choice. Washington's legislature only meets for 2-3 months per year[1], and they're paid about $56,000 per year[2], which sounds pretty good for 2-3 months of work...except that they're actually working behind the scenes all year long to get legislation lined up, and then falling over each other to get things done in a short time window every January. It's not an efficient or particularly effective system.

[1] https://leg.wa.gov/legislature/Pages/Overview.aspx

[2] https://lmtribune.com/northwest/washington-senator-per-diem-...

Most state legislatures are only part time, so when not in session they have full time jobs. That's actually how the US founders wanted it to ensure that politicians stayed "of the people" and couldn't escape the effects of their governing. "They will never vote a rod against themselves" (can't remember who said this so take with a grain of salt but it's stuck in my memory). At some point though the Federal legislators decided to give themselves full time jobs. At this point now most probably make enough money insider-trading stocks (yes the same thing that gets a normal citizen jailed), so they probably wouldn't have to work when out of session anyway.

In some states, like New Hampshire, state legislators' paychecks are a token amount like $100/year or nothing at all, and must hold an additional job.

The state legislature in WA works part time of the year, from Jan to later March I believe.