Assuming as a spare part...
Because after production ceases the cost to buy in low volumes is far, far higher.
The relative shipping costs from source are much higher for lower volumes.
To keep them on the shelf costs money.
To keep a track of them costs money.
Asus and others punt out tens of thousands of the one board thereby lowering the costs stupidly low. Make them in low labour rate countries often with dubious conditions for workers, then punt them back to us at rock bottom prices.
Once they're gone, they're gone. No parts, almost no service to provide. No warranty worth a monkey's.
So hello to cheap, low cost, essentially throw away goods.
And we eat it up. Then whine when we actually have to pay real money for stuff.
But we'll happily buy things, even sub standard, at rock bottom prices made in a sweat shop in China, Bangladesh, Taiwan, Korea or wherever... so long as we don't know about it so we can of course have a clear conscience and less to pay.
Then complain when it goes sideways or the goods are just not up to snuff.
But apparently, loads of us are all okay with that.