The cells need to come from somewhere, so animals will still be exploited regardless.

Animals are harmed when they’re displaced from their homes being converted to farmland. But it’s a one time deal for millennia worth of vegetables, so it’s acceptable to still call that vegan. I would say that taking a bit of tissue from one animal to replicate indefinitely for the foreseeable future would also fall into this category.

If we’re doing fine with tofu and other alternative proteins, why do vegans need to bother with this stuff?

We still need to cater to vegans who have food allergies and intolerances. More options available means more people who can go vegan.