The F-15 is an air superiority fighter, designed pretty much exclusively to knock the MiG-25 out of the skies over West Germany in case the Warsaw Pact ever felt like forcibly crossing the Rhine. It is a weapon permanently anchored in 1983, a Luftballoon for Major Thom when he goes Vienna Calling. It's absurdly expensive, hauls a ridiculous amount of ordinance and is about as applicable to the modern battlefield as cavalry horses were in trench warfare.

More than that, most of the F-15s we fly are F-15Es, which are air superiority fighters repurposed as light bombers. They're really useful for ploughing through skies you already own and dropping phosphorus on enemies that can't do jack shit about it. See: Iraq. We've flown our F-15Es so far beyond their operable envelopes dropping nasty shit on Iraqis, in fact, that most of them have been grounded (their kevlar/carbon fiber components, which can't be non-destructively tested, are beyond their operational envelopes, leaving open the possibility that your lift surfaces might delaminate and fluff somewhere over Sadr City, which is the exact opposite of what you want to happen). Despite this fact, we aren't building more of them.

The F-15 is such an undesirable weapon on the modern battlefield, in fact, that most of the iterations it's been through lately have been export models that we've failed to sell to other countries. Greece bought the F-16 instead (it's half as much; the F-16 was originally developed because the F-15 was so damn expensive and initially could be had for one fourth the price). Korea has some. Singapore has some. But you know why Saudi Arabia really wants them?

Israel has some. They bought 25 a while ago but they, too decided that the F16 was cheaper.

When Saudi Arabia takes delivery of their latest batch, there will be more F15s in the Persian Gulf than in the USAF. Which is appropriate; it's the perfect aircraft for airshows, stamps and sending to within inches of national boundaries in "death to the Infidel" saber-rattling exercises. For modern conflicts? Yeah, you'd rather have a few Tomahawks or Silkworms and a steady supply of Reaper UAVs. They're cheaper, more effective and somehow, you can fly a drone over places with less of a fuss than dual Pratt & Whitney afterburners.