The title is pretty much the entire question: why are squash & zucchini so often planted in hills?
Squash, zucchini and other vine crops that aren't trained to grow on a support do better on a raised bed and allowed to ramble down. The biggest reason for mounding the water drains away from the plant and its fruits inhibiting rot.
I use mounded beds ALWAYS. Not a plant do I plant will be on level ground with the exception of trees and lawns. Trees and lawns need to have undisturbed or compacted soil to sit upon so they don't sink over time allowing soil, mulch to slump down on the bark of the trunk and lawns won't have puddles, better soil to seed contact.
My ornamental plant beds, all my vegetable beds are all mounded or raised beds with no wood or concrete necessary to hold them up. If available I use turned over sod to build beds topping with topsoil before planting. Otherwise just the process of double digging one time only will easily fluff up soil into a raised bed. When double digging sometimes the soil gets 3' high. I've usually been blessed with heavy clay soils. Clay soils should never be rototilled or heavily manipulated. I always keep throwing decomposed organic matter onto the turned over shovel fulls of soil, chopping clumps somewhat After raking and shaping and compacting those beds will be a rational foot high. As organic matter is eaten and redistributed those beds will settle anyway.
Before planting seeds or starts, I take a sheet of plywood and jump up and down on it to firm up the soil. Using a rake one can pull up the soil on the sloped edges to lightly cover seed...I take a rock rake and firm the sloped edges by pounding the top of the rake head along the slopes.
This fluffing and firming raises the plant beds vastly increasing air and increasing drainage. Then there are obvious walkways and no one walks on the beds. No need to do the digging again. No worrying about too much water and rot. Just keep applying decomposed organic matter to the surface. I do this at least twice per year for my vegetable garden, once for ornamental plant beds. The soil organisms you are feeding will take that decomposed organic matter/mulch right down into the bed FOR YOU doing all the work from then on. Hey, I am the laziest gardener there is! I know just how much I have to do and NO MORE. Grins. But answering questions I am quite the opposite! Sorry!!
I feel that digging trenches around the base of my beds...all of them is critical. These trenches collect extra water, water drained from the bed away from the bed to the place of your choice. If you have a major rain storm these trenches make a world of difference saving your beds, plants and walks. Trenches go hand in hand making super edges for one's lawn at the same time. Trenches are 6" deep and 6 -10" wide on average. I throw the extra soil up onto the plant bed. Usually before I add fresh mulch. Trenches are cleaned up a bit in the spring.
I also never plant rows. I do mass seeding and staggered rows using as much of my plant beds mounded round beds. I do much more 'thinning' that weeding. Weeding has never been a problem especially if one plants a 'cover crop' for the winter.