Devs spend $5000 on assets because it’s a better investment than the time it would take to develop the same things in house…not necessarily because they can’t do it themselves.
Even the Unity tech demos that have been released required upwards of $1000 in assets; if not more. It has nothing to do with inability and more to do with fundamental things being completely absent from the engine. Unity isn’t a fully-featured game engine.
Every. single. time. that this is brought to light, Unity fanatics snap back with “But muh Asset Store…Unity can look just as good as UE4 it just takes more work and these 5 Assets!”