Sometimes it makes sense. Buying 2 B748 would be around 400m USD or so, plus the expansion of crew required. If you think the uptick in cargo is temporary, or aren't sure how sustainable it will be, wetleasing a couple more makes sense, even if more expensive.
Shep69 - all the big US airlines do exactly what you say. Anything small (78 seats or so) is wetleased by UA/ AA etc from regional jet/ turboprop companies. Because it is cheaper for DL to just buy x thousand block hours from SkyWest or whoever than fly their own aircraft