0) What if interdimensional travel is easier than we think, we just haven't figured out the hack yet. Ergo: they're not actually that advanced.
0.5) What if they're hyper-specialized? They're really good at crossing dimensions but not so good at conventional flight.
0.618) What if they're hyper-efficient, and sending these craft for them is like us manufacturing widgets in a manual factory? As in they've reduced the cost so much that a few trips having "defects" is normal and within an acceptable QA tolerance for them?
1) What if unreliability is a fundamental property of interstellar/interdimensional (or whatever we think this is) travel. In a physical-law type of way analogous to the uncertainty principle specifying an inviolable tradeoff between precision of position and momentum.
2) What if they're very promiscuous? What if there's 200 craft in Earth's atmosphere right now, and basically constantly for 1000 years. Assume they arrive and depart at basically constant rate of 40 per second (20 in / 20 out). For 90 years of cover up that's 110,730,240,000: or 110 billion. Assume they only crash on entry or exit. Assume that 20 craft have crashed in that time. That means they have a 1 in 5.5 billion chance of crashing. Googling estimates of airline crashes gives 1 in 1 million to 1 in 11 million. Making them 500 to 5000 times more reliable. Checks out.
I think the extraordinary thing is taking a default anthropocentric point of view, and extrapolating it to the whole universe, and having a high expectation that's highly likely to be valid.
also not necessarily the first one either
