Two reasons

1. Updates, every single router manufacturer abandons their routers within a few years, so you either keep buying new ones when the updates stop or you use open source firmware that extends that life until it's obsolete.

2. OpenWRT (and the others) expose all the features you need where manufacturers often simplify this or charge you extra for them. The hardware becomes use to utilise as you please.

I wish Netgear and others directly supported open source firmware like they did in the past.