[RPG] Why do ranged weapons in D&D list two numbers separated by a slash to show their range

I want to challenge your approach to this. As you note, the Light property is defined like this:

Light. A light weapon is small and easy to handle, making it ideal for use when fighting with two weapons.

This property isn't just there as a computer function — it's also part of the basic description. A hand crossbow is small and easy to handle. And it is, as far as crossbows go, ideal for fighting with two weapons. You may need special practice and training (as represented by the feat) to take advantage of it, but it's undeniably better than any other crossbow for that case.

Is it true that mechanically there are no current rules which make particular use of this property? Meh. Probably so. But it does not mean the property shouldn't be there, because that kind of mechanical interaction isn't the only reason weapons have descriptions.

It might have been more elegant from a "the puzzle pieces must fit together!" perspective if Crossbow Expert said "a crossbow with the light property" instead of "hand crossbow", but I don't think that really would add much of actual value.

Of course, there's always the possibility of future features which apply to "light ranged weapons" or the like.

About ranged attacks

Ranged weapons have a normal range and a long range. Under normal circumstances, you can attack enemies which are within your long range, but as soon as they are beyond normal range you have disadvantage on the attack. Attacking a creature while engaged in close combat does also impose disadvantage on ranged attacks. (PHB p.195)

Underwater Combat

In the special case of underwater combat, an attack beyond normal range is an automatic miss and an attack within normal range is at disadvantage unless it is one of the explicitly stated. This rule is a specific beats general case regarding ranged attacks and their two ranges.

[...] against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart)

This means, that nets are excluded from the water's specific case rule regarding attacks within normal range. So the long range is a automatic miss rule still applies to them.

The disadvantage for attacks with a net is not caused by its range, but by close combat

The rules of ranged attacks in close combat do still apply. You could still use e.g. a crossbow under water to attack a creature 10ft away from you without getting disadvantage due to the range category, but when you are engaged in close combat you still get disadvantage on ranged attacks (unless you got the crossbow expert feat) same applies for nets.