Because there really isn't a reason to tack a +1 onto TF2 and really the only issues you could point to that TF2 actually has (intrinsically) is that the SDK sucks and most of the problems with the game are extrinsic factors that wouldn't be fixed by declaring it a sequel.
>>724692915
Eh... it's more that CS:2 was the very very elaborate and convoluted process of a console port and update of CS:S (CS:GO) slowly metamorphosing into CS:S2.
Which maybe you could still call CS3 if you wanted to call CS:S CS2.