Amazon prime costs $119 a year, which is about equal to the interest from $5200 in a money market account. So it's not that $10/month seems like that much to me, but when I think about giving Amazon over $5K as a lump sum, it seems outrageous, like a sleazy trick, or a too-clever arbitrage.
And, you know, whether any amount paid is insignificant depends also on whether it gets you anything.
After I cancelled Prime, I noticed that Amazon was still shipping things just as quickly for free, even though they try to create uncertainty to get you to sign back up.
I've never used any other benefit of Prime.