[GingisK]

1 - Advanced craft are more likely to have problems than simple ones.
2 - The craft might be remotely controlled or autonomous, making them more likely to error.
3 - World governments just might be shooting them down.
4 - Advanced craft are not necessarily piloted by the best they have to offer.
5 - Advanced technology might not be the result of a higher intelligence, perhaps longer development times or accidental discoveries.

This list could get a lot longer. Remember that even our most advanced aircraft auger in from time to time.

[Windcatcher]

“Random” crashes could be a simple way of getting around one’s Prime Directive. “We didn’t *mean* for them to get their hands on our technology!”

[Starboard]

World governments just might be shooting them down.

************

That point is the most unlikely IMO. It would be extremely hard to keep that many people from leaking/talking about such spectacular events. If you’ve ever worked in an organization/bureaucracy you know how hard it is to keep secrets for long periods of time. Stuff gets out eventually.

With respect to your other points, it is entirely possible that our notions of what alien spacecraft are and how they operate may be completely wrong. Human experience and bias often color how we perceive things. If they exist, such spacecraft may not be of a form we are familiar with.