They're not skimping $2k one time cost on a cubicle. They're skimping on the recurring cost in rent and maintenance on a large facility in which to place that cubicle, plus the cost of that cubicle. My wife was an office manager that worked with a couple startups through expansions, and the costs of office space and office equipment is actually quite a bit higher than you might think.
That said, I still think it's generally a foolish thing to do for many companies and I personally despise open offices. I've recently transitioned to being fully remote, and a hatred of the open office plan was a good part of what pushed me in that direction.
I already have a designated desk area against the wall, we're talking about adding a footprint for a couple of 5cm thick walls which is already my personal space anyway, the only difference is actually enclosing it instead of letting people wander into it because the office is already full.
That said, I still think it's generally a foolish thing to do for many companies and I personally despise open offices. I've recently transitioned to being fully remote, and a hatred of the open office plan was a good part of what pushed me in that direction.