Because when Zoom gets a subpoena, they want to be able to turn over your information to Law Enforcement. It's as simple as that.
And I guarantee you that the E2EE has an escrowed key that Zoom has access to.
I would not trust their encryption. For starters, your client does not control the key. Zoom controls the key. In true E2EE, the moderator would generate the key at the start of the meeting, and the key would not be held by zoom.
From their whitepaper:
https://github.com/zoom/zoom-e2e-whitepaper/blob/master/zoom_e2e.pdf
"When a Zoom client gains entry to a Zoom meeting, it gets a 256-bit per-meeting
key created by Zoom’s servers, which retain the key to distribute it to participants as they
join. In the version of Zoom’s meeting encryption protocol released on May 30, 2020, this
per-meeting key is used to derive a per-stream key by combining the per-meeting key with
a non-secret stream ID using an HMAC function. Each stream key is used to encrypt
audio/video (UDP) packets using AES in GCM mode, with each client emitting one or
more uniquely-identified streams. "
So, this is a bit useless.
They even admit it.
"This current design provides confidentiality and authenticity for all Zoom data streams,
but it does not provide “true” end-to-end (E2E) encryption as understood by security
experts due to the lack of end-to-end key management."