I'd say it's a collection of a few things that have been mentioned.
First is that it's kind of like homework. The same thing happens when players craft ten-page backstories for their characters. You're requesting effort on the part of the other person. Sometimes that's something people are willing to do, sometimes it's not.
Second is that the effort may not be necessary. Or at least it may not be perceived as such. I've run my fair share of adventure paths, and the player's guides, while helpful, aren't necessary. Most of the time, folks can get by without having read one, and maybe making a tweak or two to their character at the start of the game.
Third is that there's something a lot of players find off-putting about restrictions placed on the one input they have to the game... their character. Limiting their selection (or, again, even just the perception that's what's happening) is often viewed as a negative. Only these races, only these backgrounds, etc.... some players just instinctively balk at that.
Fourth is that depending on the system... and my one experience with Starfinder says that it falls into this category... character creation is already more complex than it needs to be. Adding another document to refer to may just seem annoying.
Fifth is kind of tricky, and it may not apply to everyone, though I know it applies to me. It's this: if the story is already largely set, then just give me my role. Or my motivation. Like, don't hand me a book that has a bunch of predetermined elements and then act as if I'm really creating a unique character. I'd rather know what the game is ahead of time and then play that instead of expecting something different. My preferred game wouldn't be an adventure path type game, but if that's what's on the menu, then I want to know that. My 5e group is currently playing the Temple of Elemental Evil reincarneted by Goodman Games. I made an elf mage. The DM provided me with a goal of finding a pair of elves that had come to the area to investigate the temple. Cool, I can play that. I don't need to read a bunch of other pages of options and so on. My choices are limited... so just go ahead and choose one for me. For me, allowing me to pick from a handful of predetermined options on how my character connects to the events in the story is more an illusion of choice than an actual choice.
The best thing to do in this case is to have a true session zero. Discuss what's in the player's guide and craft the PCs together. I've found it's always better to do so no matter what kind of game you're running. You can craft the group as well as the individuals. You can use the stuff in the player's guide or come up with equally compelling stuff that you can explore in play. The players may have more of a chance to have some input on the game, as far as an adventure path will allow.
Another way to try and mitigate this is by engaging in between sessions in a more manageable way. Something like text messages or posts on a service like discord can accomplish a lot, and work in a way that's not as time consuming as a player's guide may seem.
In the end, I find that player's guides are kind of a tease... like, wow this game is going to allow us to make a lot of choices to determine how it goes. But it's not... it's an adventure path, it's gonna go the way it goes. Or they serve as a reminder how the GM is going to take your choices and fit them into the established story where possible... like seeing the magician shove the doves up his sleeve.
I imagine for most people who don't want to bother reading a player's guide, it's some combo of the above reasons.