Like I said - I don't agree with it either but it is what it is.
Component speakers that have separate woofers and tweeters are normally round, not oval, thus the 5.25" woofer (or mid-bass) speaker. Crutchfield gives you an adapter to fit it to the 5x7 opening.
Just get the 5x7/6x8 coax speakers if you don't want to bother with mounting the tweeter. Or get a shop to install them. Having the tweeters close to ear level makes a huge difference.