It is very much a thing. Most (fit) human beings are somewhat concave at points around their waist, have some level of squishiness in their sides and lower back

It is very much a thing. Most (fit) human beings are somewhat concave at points around their waist, have some level of squishiness in their sides and lower back, and don't particularly enjoy embedding a drawstring as far into their flesh as it will go. Not to mention the leg holes, which don't give you nearly the same purchase, but don't have drawstrings and wrap around relatively soft flesh. If you want to grab someone by the speedo, you can grab someone by the speedo, with the best grab going through the waist and a leg hole, give you complete control over your victim's waist. It's why there are different suits for water polo vs swimming. The water polo ones can be put through a lot more abuse than the swimming ones, which put under the same conditions have a tendency to tear in half, and they have less stretchy waistbands to make grabbing harder (but far from impossible).