Because the "plain text numbers" in front of them aren't plain. Tooltips are expressed in terms of Potency. Well, what does Potency mean? Nowhere in the game are we given a definition for Potency. Logic says bigger is probably better when it comes to DPS/HPS but we still don't exactly what we're dealing with.
As a new player, I though Potency simply meant "damage". I couldn't understand why my attack with a Potency of 200 was only doing 67 damage. I couldn't figure out what I was doing wrong to cause my attacks to end up so weak. It finally took going to google to find out what Potency means and how it worked with my stats to produce a damage value. If I hadn't already been familiar with damage calculations from playing WoW for years, I might still have not understood.
There are other things on tooltips that can be likewise confusing, possibly because of term holdovers from other FF games that the devs just assume everyone will understand without thinking about how many players new to the FF franchise have been attracted to this game and don't have that prior knowledge. I brought up Limit Break as something to be added because it wasn't until I did Aurum Vale for the first time and the group was yelling at me to "LB" the last boss adds that I became aware the Limit Break existed. I kept looking through my BRD abilities list trying to find one that might be abbreviated as LB and could only apologize to the group that I couldn't find an attack that was "LB". It was then they explained Limit Breaks to me. It doesn't help that Limit Break is listed under General instead with the other class/job combat abilities. It makes little sense to group such a powerful combat ability with non-combat options.
It makes even less sense to force players, especially new players, to go to outside sources to get an understanding of basic game concepts. The problem is the new player, especially if they've never played a MMO before, isn't going to know the right questions to be asking. Yet here we are and so it's little wonder that so much of the player base struggles between the lack of transparency within the game itself not to mention a mentor system where the majority of mentors have no desire to actually be mentoring.