Is part of their advertising budget. The people running are a captive audience they can advertise to and their logo can be on the tee shirts the runners receive.
If you wear that shirt later that’s more exposure for the money.
So the answer to your question is its the same reason you might buy space on a billboard. Instead of giving those dollars to a billboard company they’re giving it to the cause and getting their name plastered all over it. If it’s effective they’ll keep doing it.