Why are bus transfers not accepted on the subway?

Why don't the bus transfers that you can get when you pay with coins work for the subway?

Because that would be extremely labor intensive, requiring they be submitted to a human. I'm surprised to hear the buses even still take coins for that matter. If you want to free transfer to a subway, use a MetroCard.

There are forums on other sites to discuss buses...and Jeff summed it up. Use a Metro Card.

As an aside, it's hilarious when people pay for the EXPRESS bus in coins.

One other explanation came from a source close to a source: reselling the transfers. Still doesn't make the topic relevant as either explanation seems obvious, and it sounds suspiciously more like a customer service complaint/issue.