I've worked at a few companies including a several startups and my current company is the only place where summer fridays are a thing. Even then, last year, the tech department was usually the last to leave. Keep in mind that at some companies, summer fridays just means that you can dress down and wear shorts.

The companies that do summer fridays are companies where developers are working ~40 hour work weeks. These companies are usually places where they expect employees to last several years and often are profitable.

Why do companies do summer fridays? In the northeast, you typically have about 2-3 months where the weather is nice enough to go to the beach. If you've got kids, this is their summer vacation. Plus in many metro areas there are a lot of concerts and events going on during the summer. It's a nice way of letting your people enjoy their lives instead of just living for work.