When you look at GitHub alternatives you find a lot of projects that a doing GitHub in a sightly worse way but nothing really compelling. They are all either centralized services or self-hosted solutions that miss most of what GitHub has to offer.
1 - I'd lose all my stars
2 - I like the UI
3 - Other Git hosts don't look nice and I feel weird using them.
If GH was open source, we wouldn't be having these problems.