Text (SMS/MMS) is cleartext. Signal, et al., did not exist when iMessage was implemented. It was arguably the first true easy to use E2E encrypted messaging available to all, and not reliant on a user being proficient with *BSD, Slackware or Kali Linux.
It’s true it is closed source. But it just works, like most things Apple does. It removes the need to RTFM, or open a shell script, or read the man pages, or all the other esoteric bull**** that went into using GPGP or PGP before.
Most users still cannot grok PGP. Yes, Signal and others have also simplified E2E encryption, but Apple popularized and spread it to the masses first. And more importantly, gave it free of charge, and frictionlessly (unlike most prior encryption technology).
So your argument may hold merit, but equally so does the fact that Apple brought this technology to the masses.