The main reason, I think, is just that they've never thought about it. To many, while they have a vague understanding that there's a long history to the Levant, the functional history starts in 1948, and the question undermines that assumption.
But it creates real intellectual problems, too. Many Pro-Palestinians (as well as pro-Israelis) assume that a land can belong to an ethnic group.* If so, how do you determine which ethnic group it most legitimately belongs to when there are conflicting claims. Usually there's an implicit first-in-time, first-in-right assumption (e.g., in the very term "First Nations," which has become standard terminology in Canada). If we apply that standard and look at certain parts of the Levant (although not all of contemporary Israel!), the reflexively pro-Palestinian position is harder to maintain.
And nobody so far has ventured an answer to my follow-up question, how long do invaders have to occupy the land to gain legitimate group title. Obviously, even if we accept the premises of group title and eventual legitimate claim of another people's land through adverse possession, the question of how long it takes has no clear definitive answer. (And we're hindered, not helped, I think, by the impossibly complex history of identity groups forming, invading others' territory, and then reforming in comply ways into new identity groups).
But it's also a tough question - maybe a gotcha question - because merely to try to answer it is to admit both that at one time the Arab occupation of parts of what is now Israel - say the territory of the kingdoms of Israel and Judah - was unjust itself, and that there is some timeline through which Israel - if it can hang on - will regain legitimate title. Neither are things the Pro-Palestinian side is going to be comfortable addressing.
An alternative - which nobody has yet tried - is to junk the group title claim, and emphasize living Palestinians, or their immediate descendants, who had land literally stolen from them. But - not that any of these folks have thought about this approach that deeply yet - this approach also creates problems for the pro-Palestinian side. Ultimately it cannot logically require anything more than reparations. Israel could effectively declare a policy of eminent domain, compensate prior owners or their immediate descendants, and that's the end of it. Even with a right of return there need not be a right to reclaim property (although a right to compensation is real, and I'd argue is an obligation on Israel).
Also, being an individualist approach rather than a group approach, it doesn't help privilege the claim of either group, but privileges the claims of all individuals who live there, especially those who were born there. From a strong individualist pro-liberty perspective, anyone should have the right to live in any territory they damn well please.
(Obviously that's not a popular perspective in general, much less on either side of this territorial debate.)
The question has mostly functioned as a debate stopper, unfortunately. But I think that's because nobody is used to thinking about it and its implications for the present conflict. Nor are the implications simple and straightforward enough for people to quickly latch onto an answer and solve the issue. But if it has stopped debate because it actually has caused anyone to think, then I think asking it is worthwhile.
*As a methodological individualist and anti-collectivist, I think this is a very dubious assumption, but it's a foundational concept in the "Israelis are invaders/occupiers narrative.