The ITs are player-controlled, so the OP probably doesn't want the player to annihilate everything in their path using Infested Terrans, so giving ITs 9999 damage could be unreasonable.

There is a slight issue with blowing up the ITs when they get near the building. First of all, since it's location-driven, you're using a bounding box. This means that at the corners of the location, an IT might not be touching the target building, but it'll still blow up, since the trigger will still run. There's also the possibility that a player just wants an IT positioned next to a building, without attacking. (But no, I don't know why a player would do that.)

You can significantly improve the accuracy of the location bounds vs. the building edge when you use 2 locations per building:

do you mean, you point the building in those 2 locations (1 and 2) to the same illegal location? I tried it but there was no difference. The hovering building still keeps moving a little before stopping, same as using 1 location.

Also I cant set 9999 damage, I still need them for other things so they should have the normal damage.

What?
I was referring to DavidJCobb's 2nd paragraph:
specifically the red part.

For moving the building you ofc need only 1 location.

If you have enough extra players, you could utilize that. Have P7 and P8 unallies, but P6 and P8 allied, so give the infested terrans to P7, the buildings to P8, and the rest of the units to P6.