It is called unskilled because there is no particular set of unusual credentials, rare talents, or uncommon training required. The entry bar is very low in that respect.

It doesn’t mean, and has never meant it was easy work or anyone could survive doing it for long - rather that the necessary skills (for the majority of the population who doesn’t already have them by dint of existing) can be taught in about 5 minutes by someone not trained in teaching it to you.

Ditch digging is ‘unskilled’ because anyone who can figure out a shovel can do it, and theoretically any random passerby could do it (for a time) to a competent degree. This is unlike a ‘skilled’ trade, or craft such as electrical work, surveying, etc.

It’s something very few people could actually sustain for very long - I spent a week ditch digging before when I was younger - but part of why the pay is terrible and the work is hard is because any rando who needs some cash this week can do it. There is a ton of competition keeping prices low and conditions hard.