Why is my mobile speed score so much worse than desktop?

A much lower mobile score is normal and expected - PageSpeed Insights deliberately tests mobile on a slower simulated phone and a throttled connection, because that reflects how most people actually browse:

• Weaker hardware - a mid-range phone has far less processing power than a desktop, so heavy JavaScript hurts more.
• Slower, less stable networks - mobile data is slower and patchier than office broadband, which punishes large files.
• The same images, smaller pipe - an oversized hero image that is fine on desktop becomes the bottleneck on mobile.

The mobile score is the one to take seriously, because the majority of your visitors are on phones and Google ranks on mobile first. Optimise for the mobile score, and the desktop score takes care of itself.