You said you 5-star instead of tip, as though giving them 5 stars is something special you've given them for good service in lieu of a tip.
That would be great, but uber views their drivers as having failed if they get an-y-thing below 5 stars. If you give them a 4-star rating, you're increasing their chances of getting fired. It's a bad system.
Knowing that, if a driver is adequate, I give 5 stars. If it's a little sub-adequate, I just refrain from rating. Only if the ride was one that compels me to think other strangers should be protected from having to ride with the driver do I rate sub-5-star.
The only way to reward the driver for being better than adequate service - if you're compelled to reward - is to tip.