Why is the US military not using their reversed engineered flight tech yet.

Today the Military grounded their Osprey as another one crashed, it seems to be due to metal fatigue or just alloys not up to the job.

Why is this a thing with their technology – alien tech yet cant make a reliable alloy part. Why are they releasing “stealth” bombers that still rely on Bernoulli’s principle for flight if they have had anti gravity sorted for the last 30 years at least. Why do the US Military still persevere with the tech they do while so called ‘secret’ military have been using it for years.

I am not debating the existence of UAP’s – this is more about all the discussion around contractors reverse engineering and building working craft. It just doesn’t make sense they are still spending billions developing “old” tech.

Todays story – Air Force grounds entire Osprey fleet after Japan crash (msn.com)