I too have encountered this sort of apathy and it takes some getting over. But let’s get to brass tacks: If you cover off the following, there should be no reason left for folks not to come and not to listen, and I wonder if you and your colleagues have thought about these:
1). Include those colleagues who will be attending, to come up with or vote on talks they’d like to see given; Topics, issues etc of note.
2). Ensure that the quality of presentation has a minimum level of topic familiarity. Yes this may put off one or two folks, but if the ultimate aim is to ensure interest and the passing-on of knowledge, it’s a trade-off worth making.
I’d be surprised if no-one had any take-homes from these talks. I once gave one on a $500k website rebuild to about 20 colleagues and included a skate video slam-section to illustrate what happens if dev’s are not all singing from the same hymn sheet - many people just took that away, but I heard on the grapevine a week or so later that one new guy loved the talk and took a lot from it. Takehome here? Seek Feedback and improve the talks and what’s talked about.