And pee goes…where? Fines from the bedding will sift into your pee (errr, pea :lol:) gravel, holding urine and then clogging drains. Removing the lot of it to rinse the organics from the gravel and unclog the drain would be a huge undertaking.
The stall fabric options that use a gravel drain bed separate the drain bed from the bedding for a reason. Organic materials like bedding and inorganic materials that need to drain don’t work well without a barrier between. You could do stall skins, but at that point, you’d likely be spending more than mats, so…why?
The benefit of concrete under mats is the level surface that doesn’t deform. There’s no benefit to having concrete under gravel or sand