Why aren't older video games ported correctly?

Take the SEGA Genesis/Mega Drive, for example. Most of those games were built upon the 4:3 television aspect ratio. With televisions being in the 16:9 aspect ratio, ported games now either have to be stretched majorly, or they must be in their original format, but with black bars surround them.

I'm thinking that the next generation of consoles should allow for a change in format. 4:3 games can be in the true 16:9 format without being expanded. The only difference is, more of the screen can be seen on the left and on the right. Also, there's even larger televisions on their way! They are in the 21:9 aspect ratio, which could allow for much more space.

Why aren't they ported to match modern televisions, though?

I really don't think it's as simple as you think it would be to make something like that in a different view perspective. I don't know the technicalities myself but you'd probably have to rework a large portion of the game's code for this, you can't simply move the side borders out to see more.

I imagine portability of the game application is a major factor which is probably why C++ remains a popular language after multiple decades.

I don't think it's possible with sprite based games since those have fixed resolution and view area just like the plethora of PS1 games with pre-rendered backgrounds, but for 3D games it's totally possible and so easy it's only a matter of modifying a value in an ini file.

The PCSX2 can play every PS2 3D game in native 16:9 because 3D games are rendered in real time.

The short answer is because they aren't usually "ported" to the new platforms. A lot of the Genesis/Mega Drive game collections that Sega puts out are actually just the old games running with legal emulation. There are a few instances where they really re-work a game to run natively on a PC or modern console, but those are pretty uncommon compared to the easy path of just grabbing an open license emulator and bundling it up with a for sale sign.

Dungeons and Dragons: Chronicles of Mystara is an example of one of the games they did right. They didn't change the aspect ratio of the original game but they added side art borders, achievments, etc.

Regarding AR in older games, technically it would be possible to re-write a game to support 16:9 but that can have more effects on the games than just making more screen visible. In vertical scrollers like schmups, they would have to re-do the level artwork because the original artwork will only be as wide as the 4:3 frame. In side scrollers, the game balance can be affected by changing the AR. Side scrollers often only scroll in one direction, and the left border of the screen is often a boundary and chaning the size of the play area might make the game too easy. Now you can see charging enemies from farther away instead of having to react quickly when the come into frame. How do you deal with boss battles where the battle scene takes up exactly one 4:3 screen in the original? It seems easy, but once you actually dig into it there are a lot of decisions that need to be made about how to handle different elements of the game that need to change.

-Byshop