Why not? Problem with believing aliens inevitably exist out there is that there will be all kinds of them. Those that would avoid us and those that wouldn't. For whatever reason we might not even understand. As for ways to travel between stars we can already think of some. Generational ships for example. May take a long time to reach the destination but should work. So maybe the question is why haven't we seen any of them yet?
Because we live in a redneck area of the solar system. We're BFE. There are areas of the solar system where the distance between stars is much smaller. A generational ship would turn to dust before ever having reached us. We can't even solve our most basic problems, what makes you assume that aliens are any better at it?
Same reason. If life is abundant, some of it somewhere probably figured it out. And then add to that how long they could have existed before life on earth appeared. Aliens could have had a long time to figure it out and spread almost everywhere. Or become extinct and give way for others to do it. And by the way, if the generational ship could reach even a small fraction of the speed of light, relativistic effects like time dilation could help a lot. The ship and the crew would experience less time in transit than the distances between stars might suggest. Also it's galaxy not solar system.