Why so many small side-by-side airplanes have split windshields?
 

Is it to add extra structural strength?

Or because they couldn't form its entire curved shape from a single piece of flat lexan or whatever (flat) material they use?

It's awful as it remember reading that Europe has a regulation banning more than 2 A-pillars. That means windshields can't be split in the middle.

I think it's a combination of both...

The aluminum body is stronger than the glass, but still very thin, the larger the glass is (with no pillars between) then the more reinforcement and weight necessary around the window frame.

Modern glass technology makes it easier, but airplanes are slow to change. It was harder to make an "any shape you want" glass in the 60's. Newer airplanes can and do have more expansive single windshields alot of the time that i've seen.

The more appropriate answer would be I think "they had to up until the 60's" after which the auto industry in particular started pushing the envelope. If you can show alot of airplanes both designed and built after the 60's (aircraft like the Piper Cherokee were first made in 1960 and are still being manufactured TODAY, they wont change a design because things like FAA certification have to be redone for even small changes) I might scratch my head too, the above is a guess but probably a good one.

The manufacturer doesn't know how successful a model will be when they are designing it.
That means the required tooling is kept as simple as possible, and when the design affects the complexity of the production tooling, the design is made simpler.
Building big, complex curvature aircraft windshields with the required strength and thickness isn't easy.

You see it all the time with concept cars as well.
Designs elements that are too complex / costly for production, vaporize in the final customer version.

How many Beech King Airs or 1900s are around ?
And just how many Starships ?

Light aircraft fuselages flex a lot. Split windows minimizes stress on the windshield.