Why is the speed brake lever not a "spring" one that returns to the default position when released?
Indeed spring-loaded or not, the pilot will keep a hand on it. But in the latter case, they can still use that hand, and return to the lever, without abrupt changes to the plane's pitch/configuration.

An example busy scenario is a rapid decompression descent.


Source: YouTube; annotated

If you continue watching the video past step 4 (deploying the speed brakes), there are other things the captain attends to with his right hand: pointing something out, making a call to the cabin, fine tuning the mode control panel, putting on the oxygen mask, inputs to the CDU, etc.