The answer to your question is this. People buy on ebay because they can. The stuff is there and usually less expensive for most modern and living players than most private signings or shows. For deceased players, it is much more convenient and sometimes less expensive than going to shows and Auction Houses.
The direct enemy of fraud is education. I have been collecting autographs since 1978 and have put a significant amount of time into educating myself about autographs. I have never ever claimed to be an expert, but I do feel very comfortable being able to identify good from bad for many player autographs. Armed with this knowledge, I can very easily sift out 99% of the bad autos on ebay of the ones I am interested in. If you, or anyone else, has not put the time and energy into the education, then it is very easy to be defrauded. I agree with many of your statements about the TPAs, which is why I only use them for items I am about to sell, if the cost of the process is less than the price difference I can get for a certed auto. Obviously, for many of the above players, the items are, for the most part aren't worth it monetarily.
I am not against getting autos and shows or through private signings, but only for specific items that I can't buy one of somewhere else. If you need a specific card from a specific set that isn't around, I can def see paying a premium to get it signed. I paid $800 for a Muhammad Ali signing to get this magazine signed. Why? Because there was no other way to get it done.
On the other hand, why would anyone pay $395, Koufax's last private signing fee(2015), for a plain Koufax signed ball? You can get good Koufax signed balls on ebay for at least $100 less. Obviously, if you have no idea what Sandy Koufax's sig should look like, you should absolutely not be buying one on ebay, or anywhere else for that matter, except a show/signing. I can't tell you how many Derek Jeter and Mantle balls that a certain member of this board has bought off ebay cheapy simply because people don't know what they are supposed to look like and have no certs.
If you choose to limit your autograph collecting to just those items you see signed in person, there is nothing wrong with that. I sleep like a baby and am totally comfortable in my experience and knowledge protecting me from bad autos. Have I ever been burned? Of course, it's a risk that is factored in when you collect autos. It's merely a matter of risk vs benefit. I am willing to accept, what I feel is a small risk, so that I can enjoy a wide range of autos in my collection. You don't feel that having deceased players in your collection is worth the risk. It's just a different way of looking at things. It's not a matter of right or wrong.