To be fair,in those days MyISAM was pretty much the only DB engine that could keep up with the traffic growth rates of the early web, because it didn't enforce constraints. It was optimized for reads, not writes, and was fast enough to hold all state in one DB server for heavy traffic sites. Combined with stateless PHP and memory caching it was the best tool to use for this use case.

That it wasn't the best engine for all use cases and also corrupted data on disk occasionally is a different story. Once InnoDb was ready, you could just mix and match engines in one DB server as far as I remember and use referential constraints where needed.

Later MongoDB marketed itself as "web scale" due to no schema constraints at all. Fun times.