Why isn't the wing span of a certain airliner longer with the longer fuselage versions?
There are two main reasons:

It is easier to plug in a fuselage section than a (swept back) wing section. If you notice, one thing that has been kept constant across models is the fuselage cross section. Basically, the manufacturer can make another fuselage cross section and add it to the exiting ones.
If a wing section is added, on the other hand, the aircraft characteristics will change appreciably, in addition to the difficulty in doing so. If you are adding wing section, quite a lot of additional modifications have to be made (like control surfaces, etc.) which will increase cost, eliminating any cost savings the manufacturer could've had by using common parts across models.

The next, perhaps more important reason for this from airline viewpoint is that it will change the aircraft design group. The following table is from the FAA AC 150/5300-13 Airport Design (The ICAO definitions are more or less the same):



Image from FAA AC 150/5300-13 Airport Design
For example, the Boeing 737 and Airbus A320 have wingspans at the limit of the design group III (B737 has 117 ft 10 in while A320 has 117 ft 5 in with sharklets). Any increase will change the ADG, which will restrict the number of airports that the aircraft can be operated from, which the airlines won't like.
For example, when the 747-8 wingspan was increased, the aircraft category was changed (from ICAO category E to F, equivalent to V to VI) and compatibility studies had to be carried out to permit operations in airports were 747-400s are already in operation.
The airlines are perfectly happy to use different operational procedures or sacrifice some other characteristics (like range- for e.g. B737 MAX range drops around 200 miles between MAX 7 and MAX 9; aircraft rarely operate those distances anyway) for having the same aircraft operate in a large number of airports and reduce maintenance burden by having common parts across models.