Because you're overeating Dybala probably. Juve wants to sell him as he doesn't fit in to their plans, and he's been pretty meh for a while. Quality player on his day, but he needs to go in a system that really suits him. If we play a 4231, he would fit in really well as the 10, but with our wide players, it would probably be way too attacking. He's not a world class player and this deal wouldn't be "too good to be true", as there would be a lot of risk involved for us and it's not a position of direct need, but it would have the potential of being a brilliant deal. Believe it or not, Lukaku and Dybala are valued at just about the same price, so it's fine from that perspective.
Anyway, I'd obviously be happy with the deal, even though people massively overrate him based on what hes actually shown, and rate him more as what his potential is. That potential could be a quality attacking system though.
Also, in terms of our attack, if we let Spurs get him to replace Eriksen, with Arsenal getting Pepe, our attack would be miles behind those 2 and City/Liverpool. Dybala would help bridge that gap with goals, creativity and his movement and ability to do something out of nothing. Also wouldn't hurt having a good friend of Pogbas here to keep Pogba happy...