I should think because lids do make things more convenient.
Believe it or not, even those of us who aren’t opposed to getting rid of lids (and straws, single-use plastic bags, etc.) recognise what they offer. As policies and laws around the world change, I have had to make adjustments that render certain everyday situations less easy than they used to be (e.g., having to take shopping bags with me rather than just grabbing one when paying for my items). These are shifts that are going to happen whether I like them or not; I accept them for environmental reasons but also because I recognise their inevitability.
Regarding lids specifically, I do think there are times when not having them can prove not just inconvenient, but even dangerous (when carrying a hot drink, for example). I hope we move towards non-plastic / biodegradable alternatives for such situations.