Multi-tenant design is a huge win in terms of reducing developer toil and expense.

Many customers will have a tiny amount of data. For those customers a dedicated database is huge amount of overhead. There may not be any single customer who it makes sense to allocate dedicated "hardware" for.

Sure you have to deal with a one-time pain to shard your thingy, but you don't need to pay for tens-of-thousands of individual database servers, write interesting tools to keep their schemas in sync, wrangle their backups, etc.