Because you get the benefits of those who do hack, simply by installing a recent stable version of OpenWRT. It's no harder than upgrading and configuring the vendor's firmware. If you tie yourself to vendor firmware, you're tying yourself to 5+ year old kernels and all the associated security and performance and stability problems and a much more restricted feature set.
The commercial vendors do an absolutely horrible job of supporting or maintaining their products and they tend to get a lot wrong with the initial software release. The OpenWRT community does a great job of putting out a solid product that works and has sensible defaults, but you can only get the full benefit of their work if you buy hardware that is open to their hacking.
This seems to contradict itself - how does it pay if you're not going to be hacking?