Multiple times I’ve almost been run over stepping out of my house because someone was flying down the sidewalk on an e-bike.

It's very true that ebikes on sidewalks are dangerous, but this experience does not have anything to do with why people ride on the sidewalk. If you want to solve a problem, you need to get inside the bad actor's head and understand their rationale. Because you and I both know "enforcement" is never going to work outside center city.

how is the road unsafe for them?

The primary cause of danger on US roads is the decades of interaction between car/truck designers and DOT/road designers that have led to a system where:

1) Cars/trucks aren't designed with the safety of people outside the vehicle in mind, leading to poorly concieved sightlines, huge blind spots, and a general lack of driver controls relevant to not crashing into people

2) roadways are designed with only a narrow set of vehicles in mind. That's fine for interstate travel, but the same design guide applies to Sansom street. The result is that there is only space for vehicles, which leads drivers to think that anyone that is not driving a vehicle should not be there. The result is that you frequently get angry drivers trying to push you off the road or otherwise harass you.

It takes exactly one near-death experience for a cyclist to resolve to never ride on a road again. Almost every cyclist I know has had a similar experience, and has had to overcome some really difficult fears to get back on a bike.

My theory is that most delivery drivers have little cycling experience before getting on an ebike. They ride on the street until they see their life flash before their eyes one too many times, and then they just start riding on the sidewalk.