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If I were making enough via witness income to justify it, I would.

Then I guess that's the answer to your question in the title. There aren't many full nodes because people aren't making enough to justify the cost. The amount of RAM it takes to run a full node is exorbitant, it's a serious flaw in Steem (in my opinion). Instead of focusing on SMT's, they should be focusing on ways to reduce the RAM usage because the ram use it too damn high at the moment. Not to mention the space you need and you'll definitely want a SSD.

Thank you for this. And thank you bernie for pointing this out. Witness discussion/ "promotion" transparency/ data, should be on the top priority-list for steemit. It's kind of a strange paradox of "decentralization--" things can become very insular, making dissemination of information diffcult--all the while, a blockchain, by its very design--ensures some form of "solidarity" or uniformity "across all nodes" (as has been explained above--if I 'm understanidng correctly). This, imo, is the problem with DPOS. At least with EOS they kind of seemed to have some sort of "voting"/ procedure to determine block producers--although I could only imagine what a put-on all that could have been. Unfortunately, I'm not close enough to the tech to truly know. But I'm doing my best to learn.