Most of the time the issue is the "9 women can't have a baby in a month" problem where adding more resources is not going to make things happen faster - in fact it may slow things down.
But the business doesn't understand that software is not like construction where adding more people really will get that ditch dug faster.
At its core software is not making a thing - it's inventing a machine that makes the thing.
If you're Little Debbie and you have a machine that makes cupcakes it's hard when the business comes in and says, "now it needs to make fudge rounds." And no amount of extra people working on the problem will get that machine retooled any faster.