define often? You would have to know how often they visit to say they crash often. What if they have been operating here for thousands of years and we have a crash or two. Would that be often?

Stealth? How do you know anything we have can track them very well? What if they have to let us track them, or there are certain operating modes where they are susceptible to radar and other times they are totally invisible.

Making any conclusions about why they would not be tracked or wouldn't is pure speculation and worse based on our own capabilities. 'Stealth' from radar might be incredibly easy for them.