More like you can run Apache except in specific circumstances. People will put up with a lot if there's no alternative.
The alternative is to write it yourself or commission it, so let's be honest, it is about the cost. When you don't know what something is about, it's about money
A (very) long time ago the Apache developers could have gone down this route.
> You can only run Apache under very specific circumstances!
Or memcached:
> You are only allowed to run a memcached server if you're only caching your own website!
We see how nonsensical this is