Why should I plant my squash/melons in hills?

stops the leaves and flowers lying in damp conditions thereby allowing fungal troubles

Up to the 1950's almost all farming was dependent upon Flood Irrigation to water plants. Sprinklers were too expensive and too labor intensive for field crops.

Plants were placed on hills or raised rows to prevent drowning the plant. The tradition of planting in hills or raised rows has stuck with us although it is no longer necessary since sprinkler irrigation now controls exactly how much water the plant gets.

I too use shallow dish shape hollows for plants to conserve water and have excellent results.

No idea why they have to grow on a hill. (It's supposed to be a "mound," not a hill, and that's how native Americans used to grow them.) I grew mine in containers. As far as worrying about water, I'm under the assumption you will water thoroughly that it soaks down instead of dribbles down the mound.

Then again, you want to water sparingly? Then don't grow either. They're both water loving plants, so if you're going cheap on the water, grow drought-tolerant plants, not vegetables ot fruited vines.

I have never done that.