Why? Because altcoins are often inexpensive and faster than Bitcoin. Especially the newer ones. Those who missed the "Bitcoin train", will find altcoins appealing. If you want to make money fast, then altcoins are for you. But be aware that the risk of loss is higher. After all, altcoins are widely untested and volatile. If you want to play it safe, choose Bitcoin instead.

I'd expect altcoins to exist as long as Bitcoin remains alive and running as usual. The day Bitcoin dies, will be the day when the whole crypto market will die with it. That is to say, most of the altcoins we know and love today will disappear in the future. Even with Bitcoin in play. The lack of development/innovation or real use cases could result in the demise of existing altcoins. Eventually, new coins will take over existing ones as people move on to the next big thing in crypto. Who knows what the future will bring?