Because I think it's bad game design.
EDIT: Also because I think there needs to at least be a reason to use a low level damage spell in a low level slot over a cantrip.
Well, again, isn't that because every low level damage spell in a low level slot, outside of a couple of corner cases, deals more damage than a cantrip?
And, well, if you're not interested in upping the damage on low level slots (since you don't want to increase the damage that casters are doing), then doesn't that mean that we should be lowering the damage of cantrips instead?
And, lastly, as mentioned before, a 17th level caster has 19 spell slots, minimum. How long are you expecting a full adventuring day to last? Before we can move forward here, we need to establish some baselines. I suggested a 20 round adventuring day. That's 4-6 encounters, by and large. Seems about right, particularly at the very high levels. So, I'm kind of confused why you think that cantrips are being used regularly at very high levels. Why would they? They deal less damage, are less effective and less versatile than even 1st level spells and the caster, by this point, has so many slots that 1st and 2nd level spells are effectively unlimited.
Never minding that by this level, the caster is walking around with more than a few magic items that allow further spells to be cast - staves, wands, whatever. Which, again, is always going to be better than a cantrip. If I've got a wand of fireballs, why am I bothering with a cantrip? I can drop six fireballs a day without worry. What would be the point of dropping a fire bolt?