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-...