While you can check seat pitch with seatguru, a lot of airlines don't allow you to choose seats at the time of booking, and some don't allow booking of extra-legroom economy seats at all. Others charge prohibitively expensive fees for booking the better seats, and sometimes the good seats just happen to be completely taken.

It's also worth noting that there's just not a lot of variety in seat pitch. It's not like I can book a flight with Long and Tall airlines!