Why does an airliner fuselage have a constant section over its length rather than a tear drop shape?
On the other hand, fuselage on an airliner is most of time bullet shaped.


No, it is not. A “bullet shape” has a flat end, which is where most of the drag is generated (at subsonic speeds). But that shape is never used on flying vehicles except on rockets where the end is occupied by the rocket engines.

Instead, airliners always have a tapered “boat” tail, sloping gently enough to avoid separation (sometimes with help of strakes). The resulting shape is as if you split a droplet in the widest point and insert a piece of cylinder. And the difference in drag coefficient between pure droplet and droplet with cylinder section inserted is very small.

Still, a continuously varying shape is slightly better than the cylindrical section. And the early aircraft did have that. Likely the last one of those was the Lockheed Constellation. The shape was great aerodynamically, but it was expensive to build, it made it more difficult to ensure all elements have proper strength to withstand the pressurisation, every time they derived a larger version they had to redesign all frames and in operation it was not exactly convenient for loading either.

In contrast the tubular section is much easier to design, easier to make and when they want to make a bigger version of the plane, they just insert a couple more frames and mount stronger engines to match the increased weight—between A318 to A321 or between all variants of B737-MAX, vast majority of parts is the same and there is just different number of frames and different engines. And it's simple to load—seats are mounted on straight rails and containers simply slide in to the cargo bay. And the difference in drag is small enough that the cheaper manufacturing and practicality will easily offset the increase in consumption.