It's not the safety. One could conceivably put a glas she'll around the burnt fuel and it would survive reentry.

But first of all, the cost is still massive, even with SpaceX. And furthermore, you'd just put it into (decaying) low earth orbit that way. Further away would be even more expensive.

Finally, spent fuel still contains a lot of energy and should be recycled. This in turn is a very nasty and expensive procedure.