May be the reason behind it is every new website will give good returns on gambling with higher bonus. So people will shift from one gambling casino website to another for higher bonus.
It seriously depends on where are you going to put your funds, i would never join X casino because they are giving me a "+10%" extra bonus, that is ridiculous.. And it doesn't make sense at all.
But most of gambler will not think similar to you and they are preferring higher bonus regardless of the reputation of gambling houses. Higher bonus must be the one of the reason why gamblers always looking for new casino to test their luck.
When a gambler is going for a new gambling house their risk will be always in double the level as basically gambling has more risk and when they are playing with new gambling house then another level of risk playing against them. Playing a new gambling house will not be a big problem when that gambling house has a responsive customer support and maintaining a good reputation in this forum.
If you're already comfortable and confident with a certain casino, then you curiosity and promotions would likely be the main reason why you'll try others out. Casinos are for entertainment and it becomes less entertaining if you don't try other variations