To answer your question directly - because many times they are right that many of us do want to buy something from some of them.
On a day when the sand-gravity is particularly strong, I actually ask the beach guard that if I miss them, to please send Neville the Patty Man, or the Fruit Lady, over to me. Granted, those are 2 of the few who do the beach selling right - they come by announcing to everyone that they are there, but not butting in to anyone's privacy. But I don't mind looking at what some of the others walking by my beach spot are trying to sell.
I agree it can get annoying being approached every five minutes when you are walking the beach - and it took me a few trips to figure out how to deal with it. Basically, it comes down to YOU having the right attitude - noone can ruin your day if you don't let them. Just stay positive, say you don't need anything if you don't want to be bothered at that moment, but with a smile and in a friendly manner. Believe me, after a short time they will remember you and know you are not a good prospect to waste their time on.
But maybe you could try another approach. Since you ARE on vacation and have some time, actually go into a few of the craft stalls during your walk and chat with them. If they can talk knowledgably about making their crafts, and have several in various stages of being made - you can be assured it's not wholesale stuff but is what they do for a living. How do you think so many of us on here got to be friends with many of the locals - we actually took the time to meet them - even if we didn't buy something from them. THAT is why so many of us keep coming back. But I agree it's not for everyone, and you certainly are entitled to your opinion - as are we to disagree.
And, like most others have said, I do disagree about calling them "scammers" - they are "hagglers". If you manage to haggle a 1/2 hour on a jet ski for $35, while I paid $40, was I scammed? I would not call it that, you were just a better negotiator. If you do think that, then the airlines are scamming everyone except the few who got their seats at the lowest discount price - their prices fluctuate daily, if not hourly. The same with hotels and concerts and sporting events, etc. There are a few trying to actually cheat you outright - which is what I would limit to calling a scam - but only a very few.