Because they're not obtrusive, so I don't give a shit. I use uBlock Origin on Firefox because most websites are practically unreadable without it these days, but I hardly notice them on Sync. There's one ad in my feed for about every 10 posts and one small banner ad on each comment section, and half the time they don't even load. I only have moral objections to ads if they're invading your privacy or pretending to be content, but these aren't fake posts and I'm fairly certain that can't get any significant data on me since I disable Ad ID on my devices.

I could pay $17 bucks a year to get rid of them. (I often tell myself I'm going to do that to support the dev, but then I forget to follow through.) But sometimes it's less effort to just ignore ads than get rid of them. Like, I could close my eyes and plug my ears every time an ad comes on TV, but it's easier to just not pay attention.