Well, a lot of gamblers may join a new platform where they never played before, only to avail offers and bonuses, but that does not mean that those gamblers would go away once the promotion is over. Actually, it depends upon the services they got during the promotion period. If the casino had good customer support, the games were fair, and the interface was attractive, some of the gamblers may even stay after the bonus period is over.
Additionally, the casino offers promotions with the intention of attracting gamblers to their platform and then retaining them. For this, they may keep offering something and keep the gamblers engaged.
Often the best way for a casino to attract new players is very generous promotions, as people tend to stick to what they know unless there is a very attractive offer. The best casinos will have great welcome offers but also ongoing smaller rewards, it's usually an indicator the casino is doing well and can afford to be quite generous with returning users. I've encountered lots of stingy and tight casinos in the past, they usually end up much lower on my list when choosing where to play - sometimes they never get visited again. A keen gambler can spot the ever so slight differences in the frequency, freebies and payouts between casinos - the more regular the better.