Tech (is that our industry?) is not unique in it's practice of hosting happy hours, especially after conferences.
People like drinking. We could ask more conferences to host 5k jogs as an alternative meeting event, and it would be novel and fun, but it likely wouldn't ever be as inclusive as a happy hour (Also, it's hard to get 5 people in a circle talking while on a jog).
Not everyone drinks or likes drinking, but as a practical matter, it's:
1. convention
2. inexpensive for organizers
3. easy to organize
4. likely to encourage mingling and conversation
Which are important goals for organizers that aren't as easily met by alternate activities.