Because those Calvinists who adhere to the Synod of Dort, which if I recall is where the “TULIP” idea originated, are monergists and believe that only an elect that are predetermined can be saved, and not through any individual action on their own.
Now on the one hand God obviously knows who will chose to love Him and who will not, but on the other hand, I don’t think this leads to Calvinism, although I had subscribed to Calvinism in the past, and I will say that Calvinism cannot be disproven as far as I can tell on the basis of the scriptural text alone, but rather, the reason for rejecting it is based, ironically, on Patristics. I say ironically because prior to the “neo-Orthodoxy” of Karl Barth, Calvinists engaged in a great deal of Patristic studies, and even coined the Latin phrase “consensus patrum,” although Calvinists do not accept the consensus patrum on issues such as monergism or iconoclasm, where the early Church Fathers clearly rejected, by a great majority, these errors. Additionally some Calvinists even embrace Nestorianism, although John Calvin did not.