Completely different philosophical foundations.
They come from very different traditions of thought, with different foundational thinkers and theorists who have very little overlap, and mostly conflict with each other.
The moral philosophies are different. Their ideas about the state are different. Liberals like to think their ideas are founded on “reason” and we can flourish by maximising our ability to reason within an “open society”.
Of course, liberalism has branched into differing sub-ideologies over time. Some of it is as reactionary and sadistic as far-right politics, and some of it has come to resemble milquetoast social-democracy: rooted in more or less the same sort of intellectual starting points. For the most part, it’s just very conservative but with a different aesthetic, and different rhetorical appeal.
Liberal politicians sometimes talk a good talk, but mostly maintain the status quo, and appeal to comfortable middle-class professionals who like the self-serving feel-good aesthetic of “progressive politics” while changing as little as possible of anything.
Their other major ideological purpose is to marginalise actual left-wing politics and activism. Liberals like to co-opt left-wing movements, and then neutering them or repurposing them for their own narrow class interests. They’ve done this with trade-unionism, feminism, and they’re now doing it with BLM.
Leftists on the other hand, want to change the word, and burn down your rotten fucking system and replace it with something better, and actually more rational and more conducive to human flourishing, so we can end evils like homelessness, hunger, poverty, etc.