answers
"But, who said the income was guaranteed, let alone being sporadic in the first place?"
It's only guaranteed "on paper" or according to simulations run, just like the mathematical edge that the house has "guarantees" a win for them in the long run. But I'm sure there are factors which could render these simulations useless in real life in some cases, or a very bad run of luck could wipe the gambler out despite a winning system.
"How many are really making a living solely on a career of gambling? Any idea?"
I would say no more than 5%, and probably much less, of the total pool of gamblers actually win, depending on the game. I believe winning has more to do with a person's psychological makeup than a theoretical mathematic edge. Of course, if you were to ask them, 100% of the gamblers would claim to be winners.
"If it was such a dependable source of great income, why the known experts are trying something else for their living and not pursuing what they profess to be so lucrative? Thanks."
Some winning gamblers are motivated by a sincere desire to help others. Others may be winning gamblers but are undercapitalized or for some other reason are unable to make enough to make it worthwhile, or are unwilling or unable to invest the time required to do so. If you could make 10,000 a year playing blackjack or 100,000 a year in the stock market, which would you choose? Still others write about systems which may have worked for the author, but are now useless due to too many people knowing about it, or because the house knows about it and makes it impossible to win due to countermeasures or barring. A few books illustrate systems which work "in theory", but where practical application in real life is problematic. Many of them, however, are in the "John Patrick" camp; they sell books because they can't win and thus must sell books because it's the only way they can make money in the gambling field.
The problem we have here is by writing a book about his winning systems, the author "lets the cat out of the bag" and makes it harder for those who use this system in the future to win. For example, everyone can't use the same winning system at the race track, because the pari-mutuel system dictates that there must be losers in order to pay off the winners. That is why blindly using speed figures is much harder now than it was 20 years ago when a lot less people had access to them. In the case of casino games, publishing a system gives the house knowledge of that winning system, potentially enabling the house to set up countermeasures to protect itself from players of this system.