to answer your question: Boeing tends to overbuild its base aircraft, in order to have the ability to stretch them should the market dictate. It did so twice with the 762 airframe. It's also stretched the 777, 757, and of course.... 737.
The problem is, the 73G is actually the base of the 737NG line due to WN
's launch, not the 736. The 738 came later, as did the 739.... each having similar aggregate costs, but lower per-seat costs.
The 736, like the A318, a direct shrink with many of the same parts/components/features (and therefore, similar weight). The 736 has exceeded the acceptable margin of seats/weight that its longer brethren share. This is a common problem with shrinks.
A319 and A332 have been notable acceptions.
The proposed 771X and A335 flopped, for this reason, before ever being launched.