They care if somebody else is doing it in enough less than 30 hours to notice for the same fare, since at that point it becomes a marketing issue. The catch is "for the same fare." As noted in another post, flying much faster than today's jets (typically Mach 0.82-0.85*) requires a great deal of additional energy. As you approach Mach 1, the required power goes up dramatically until it comes down again (somewhat, not nearly all the way back) on the other side. The added fuel consumption you need to get a 10 percent speed increase is a bad trade-off today. You could do it, but the market (enough people, on enough routes, willing to pay the price for the time savings) isn't there. That's what killed the Sonic Cruiser concept.

________________________________
*One reason today's generation of airliners is fractionally slower than older ones is that modern turbofan engines, while much more economical than older turbojets, work better at slightly lower speeds.