I assume it was originally done to simplify the language. I think it goes right back to the original DoomBuilder which called tids tags in its thing editor.
Doing this may simplify the language but, personally, I feel that changing the language after over two decades isn't particularly helpful and, actually, has the potential to confuse things a bit. If I'm talking about a tid, I know that I'm taking about a number that identifies a thing, a LineID identifies a line and a tag identifies a sector. Call them all tags and suddenly, I don't know what we are referring to.
What's more, it means that DoomBuilder is out of step with almost all the other documentation which still uses tids, LineIDs and tags and so people can end up confused and/or talking at cross purposes and never knowing what to call these different numbers when referring to them. I've been caught out a few times trying to explain something to someone because I've been talking about tids and they can't find them. So I have to end up typing something like "set the thing tag (tid)".