> More social services means homeless from less accommodating communities will move in.

I don't know why this point seems to escape people in SF, LA, Seattle, etc. If you're homeless, you're gonna want to be where the most benefits and relaxed attitudes toward homelessness are. It's not hard to understand why this problem is at its worst in the most progressive cities.

These are all cities without overly harsh Summer and Winter seasons. That makes it easier to be outside year round. The destitute homeless in less favorable regions aren't generally mobile enough to migrate to these places.

NYC has a massive homeless population as well.

I'm sure that plays a part. However, there are small cities around Seattle, with similar climates, that have very few homeless. My interpretation is that these places have less services and the police are more likely to move the homeless. Plus, while living outside in LA doesn't sound so bad, Seattle can't be too high on the list of nice places to live outside.