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But each packet is numbered, why can't one use the ACK of the numbered packet instead?

You understand this is an encrypted protocol right?

A router or other node in the network that's just moving these packets can't decrypt them, so it can't tell the "ACK of the numbered packet". The spin bit lets such a node estimate the round trip time anyway because it will "spin" between set and unset on each round trip.

Why does an intermediate node (ie. not the endpoint) need to know the RTT?

Troubleshooting seems to be an especially popular reason to want this, but I understand some traffic management strategies want to measure RTT also under normal use.

I was thinking the same thing, though maybe QUIC doesn't send acknowledgements on every packet?

I think you get this information for free as part of TCP.

That's the answer, almost all of QUIC is encrypted for privacy and protection against meddling middleboxes and protocol evolution. ACKs can be done in many ways which should enough reason for them to be protected so middleboxes doesn't stop protocol evolution.

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