One reason is because it invites an entirely different relationship with ASCAP that many cafe and bar owners aren't ready to deal with. You'll be required to pay licensing for playing pre-recorded music as well, but the rules and the amount of money are easier.

A second reason for many shops is that they don't have the space or layout to accommodate live performers, or their lease forbids it.

A third reason for bars (probably not cafes, unless they also sell wine/beer) is that in most states live music requires a different liquor license, and switching licenses can mean going through the whole approval process from scratch, or it may be a license you can't qualify for at all.

Lastly, somebody will have to act as booking agent, because those performers won't hire and schedule themselves. This also costs time and money.