4.5 hours is not a long flight. AA frequently runs planes without lieflats on 6+ hour flights. The flight in question is a daytime flight in both directions, so there isn’t much need for it to have lie-flats, anyway.

I’m afraid your expectations are a little out of whack with reality. You should be happy that there’s enough demand to even warrant the 1 widebody flight on such a short route.

Inciidentally, the 737 used on the daytime flight are fairly young. At 8.5 years, it’s the youngest type after the 787 and A321. The 767s run on the later flight are definitely ancient, though. (But do have lie flat seats.)