I can guess as to why. When I’m using my iPhone with the MagSafe Battery (on warm/hot days), often it stops charging until everything cools back down.
I got hit with that a few times today when walking outside using my MagSafe Battery (granted, it is a hot day today where I live). I didn’t originally have this issue (even on hot days), but ever since Apple did that MagSafe Battery firmware update to increase the limit to 7.5w (up from the original 5w limit), everything just gets so darn hot.
Apple (obviously) could limit the reverse wireless charging so it wouldn't get so hot... but Apple could also do that with the MagSafe Battery (but went in the opposite direction with it, lol).
I genuinely wish that I could force the MagSafe Battery to charge at a much lower wattage (rather than charging faster, but getting super hot in the process). But the firmware update (which was automatic and I couldn’t stop it from happening) just forces the 7.5w on me, lol.
And technically there is reverse charging (since the iPhone can reverse charge the MagSafe Battery when the iPhone is plugged in and charging). Obviously I know you mean reverse charging of other accessories (and without the iPhone already being plugged in). But since you can reverse charge the MagSafe Battery in that specific circumstance, maybe Apple can enable it (firmware update) in the future for other Apple accessories.