It really is though. Memory and thus data _and_ instruction encoding were incredibly important. Physical wires on the circuit board were at a premium then as well. It was an incredibly popular platform because it was highly capable while being stupidly cheap compared to other setups.
Engineering is all about tradeoffs. "Purity" almost never makes it on the whiteboard.
This design allowed for a smaller chip, and keeping backwards compatibility with the 8080.
Yet, it extended CP/M by allowing you to use more than 64 KB vs. 8080/Z80.
x86 could have easily had an IP-relative addressing mode for data from the beginning (jumps and calls already had it), but to get a pointer you can pass around to use someplace else than the current instruction, it has to be either absolute, or relative to some other "base" register which stays constant. Like the segment registers.
The 286 and 386 in protected mode did allow segments with any base address (24 or 32 bits), so your argument about extending the address space doesn't make sense.
386 (not sure how 286 works) did extend segments to a larger address space, by converting them to segment selectors, but it requires a significantly more complex MMU as it is a form of virtual memory.
The 286 did that, though they only extended the address space to 24 bits. The 386 extended it again to 32 bits.