Because you can. I would argue this is the PERFECT time to do so. When you get to pay Roman Anthony, Bryan Bello, and Tristan Casas close to a million dollars a year that saves you the money TO spend it elsewhere and build a super team without going over the luxury tax limit if you choose to continue to stay below.
Building young talent and never spending is small market team, Red Sox may not be trying to be a big market team any more but they're still a mid market team, and those guys jump in the water from time to time. You strike when the iron is hot, and that may be either this season, or next offseason.
But I agree with your last sentence, which is why I'm a huge advocate of the Sox spending big on FA pitchers (Tanner Scott, Corbin Burnes)