Because it's faster, and because you don't want to run out of room. The larger the drive, the more memory controllers it has (to a point) and hence the faster it can pull data. Those 32/64GB drives are usually half the speed of normal SSDs. People buy SSDs for speed, and if you don't have the room for the programs or you went small and it's not fast, then you don't have speed. Pagefiles, restore points, etc can all eat at that over time. And you want your programs on the SSD so they run fast. Why spend $40-50 on a 64GB now when you can spend $100 and get 4x the room?
Here's my setup. I have my OS on an old Samsung 470 128GB SSD. My steam folder sits on its own SSD, the Samsung 840pro which is 256GBs. I have two 4TB drives for general storage. If you ONLY have your OS on an SSD, then you will still be waiting, and waiting for your games to load. Or as I said you can spend a bit more now and not worry about room as much.