I'll say this as somebody who's sat through both games.
At least FFXIII-2 could be considered a game, if a bland one, rather than a tech demo like its predecessor was.
I almost agree with this. I liked FF13 for a while. But after you realize ALL there is to do is fight, then yeah, I sympathize with the "tech demo" comment.
I dunno... maybe it's just the battle system that people can't love. Turn-based that moves with such a ridiculous speed that an "auto battle" option is necessary? FFX had quick turn-based, KH does quick RPG-like gameplay, Star Ocean and Tales have perfected real-time RPGs.
Maybe the formula is just wrong?
Actually. I don't think the battle system was what was wrong with the game. After all XIII-2 made itself into a fairly serviceable, if bland, game using the same system as XIII-1.
The problem with XIII-1 was that the game did not trust you with it at all. Instead of giving you most of your basic options right away you're forced through a tutorial for each and every one of them over the course of the first 2/3rds of the game. By the time you can change your party leader and rearrange your party load out, you're right outside of Space Pope's door for the first time and one awful corridor away from planet Namek for your requisite training montage. To further drive the point home you have meaningless tutorials like the weather control devices that show up in all of one area, an unreliable preemptive strike mechanic which is almost completely random, enemy encounters that almost always just sit in the middle of the corridor and are impossible to get around or ignore, rushed plot upgrades like the three elideons you get during your training montage on planet Namek, one non-plot mandated upgrade mechanic which is so convoluted that it's justifiably better left til post game due to how inefficient and farming intensive it is, and the fact that the main character advancement system (i.e. the Crystarium) is so heavily contingent on the progress of the plot that you can't even get full access to it until the post game.
The problem with XIII-2 was that the game felt low effort and presented low challenge and a high encounter rate. The only differences in time periods within the same area are where the artificial barricades are placed and what lighting/particle filter they're using. You get to make no meaningful influences on any time period since the plot would break into a paradox ending if you didn't do what was required of you. And even the non-linearity felt contrived to a certain respect due to how the plot mandated you into everything.
The worst part of both games though is just how little agency and relevance to the plot you're given. In XIII-1 every time you fight the Space Pope, he simply gets right back up again, brushes himself off, and tells you to :frogout: until you're stronger until the third time where he gets tired of this and fusion-has to the other Robo God as per his original unhindered plan. Beating that Robo God just results in a TPZ (total party zombification) save for the real stars of the show Fang and Vanille (Lightning and the others only really existed to provide moral support) then a literal Deus Ex Machina occurs, Fang and Vanille fusion-has into the final form of the main character from the anime S-cry-ed and bullshits a happy ending out of their combined asses.
At least until XIII-2 comes along to retcon it into a lovely fanfiction where Lightning is now a Valkyrie Profile rassalin' with the biggest loser Squeenix has ever set upon players as a final antagonist (yes, even worse than Shoe-in from X-2) while Sora from Kingdom Hearts shows up to team up with Serrah-chan and a goofy Moogle. Now they go along solving paradoxes, and helping others with their paradoxes, and paradoxes, paradoxes, paradoxes, paradoxes.... until they finally throw down with purple Sephiroth wielding Soul Edge in a form fitting full body suit (after like the third or fourth time) and regardless of your choices he commits suicide while simultaneously shitting the timeline and reality up as per his original plan which was impossible to stop since he had already won vis-a-vis paradoxes.