I don't think Multipath TCP has been tested in enough environments to become the default yet. It's compatible with TCP, yes, but it's mostly useful for e.g. mobile devices that have multiple links like Wi-Fi and 4G, and it lets users to maintain TCP connection to a certain service even when moving across networks. Go seems to be server-oriented first, and there are some potential downsides to multipath TCP in a datacenter environment (e.g. potentially higher CPU usage, etc).
From what I heard the reason for not defaulting, its not yet acceptable across different platforms esp windows and most who’ll need this are data centers 5 years its too long, since linux kernel has accepted mptcp