default TCP Maximum Segment Size
The short answer is because of RFC 791 which reads "All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole or in fragments)."

If you take 20 octets for IP and 20 octets for TCP then you're left with 536 for MSS.

Now why is 576 chosen? Mostly likely the fact that machine at that time wasn't that powerful and some reasonable minimum number had to be agreed that would be supported across all of the devices regardless how low-fi they are.