I'd say a few factors. Regardless of his quality, he is an attacking midfielder/winger. Doesn't seem our way to spend 100m (would be a record fee) on a position we're not overly lacking on, unless there's a departure first, which hasn't happened, despite rumours of Bernardo being the one on his way out.
If we do spend that, and get Grealish in before a sale, it would make our position weak with whoever wants to buy Bernardo imo, considering La Liga clubs are not doing so well financially, they could simply turn around and say a loan + option to buy for €50-60m odd, or just offer lower than what we value Bernardo at. Just thinking out loud obviously. I have stated before I would absolutely love Grealish in our side, just seems that it's well far away than what we might believe.