Sure, because those tenants signed contracts with perpetual leases (as also understood by the landlords who provided the contracts). Evicting in SF means you can't turn your property into a Condo (only a TIC) - so to maintain the value of the condo conversion option, landlords pay tenants to leave. The buyouts are proportional to the rent difference so it's not surprising that someone who's rent is increasing from say $1k/month to $4k/month would need a large sum of money (that's taxable!) to be convinced to leave.

Call the buyout $100k -- best case scenario, it's taxed as capital gains so you'd take home $85k. At $3k/month increase in rent, you'd be looking at a little over two years before you're below water.

Housing in California is broken in 1,000 different ways but the tenant buyouts seem to be pretty minor in the grand scheme of things.