Because it has to.
The F-15EX is a pretty big step up from the original F-15E's, the same with the F-16V compared to the F-16C. Depending upon your sources, both are running about 20 million-ish dollars less per copy than the F-35A with a lower cost per flight hour than the F-35A. Maintenance isn't as intensive and it takes less time to spin up -15/-16 maintainers than -35 folks. The F-15EX is a bomb truck, I'd have to look up the loadout differences between it and the -35, but it is massive.
So, you need aircraft, better aircraft than what you have, and the -35 is more costly and in some respects less capable than a modernized 4th gen fighter.
This whole F-35 debacle, that's what it is, has forced the Navy and AF to rethink their Wing compositions. A few years ago the Navy claimed the entire FA-18E/F force would be gone by 2035. That's not happening, though the last F-18 will roll off the line in 2025.
Everyone knows 6th gen a/c are being tested, 5th gen is running way over budget which drives down acquisition numbers, so you need something to bridge the gap until the 6th gen stuff comes online. Upgrades and service life extensions are the only way to make that happen.