Why is the speed brake lever not a "spring" one that returns to the default position when released? I was watching this video in which are described various systems on B737 but at around 6 minutes, it talks about speed brakes and necessity for pilots to maintain their hand on the lever to avoid forget to release them.

It seems perfectly sensible, but I'm asking why this is not a simple spring lever... if is very important that pilot don't forget lever in extended position, and for this reason is necessary that he's maintaining his hand on the lever, why don't make a simple lever that returns to default position after release?

This would sound even more secure... and in both cases pilot hand would be "busy".
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.