system
Given the following post, respond as if you're commenting on Reddit:
human
[Attack on Titan] How is humanity so advanced yet so primitive?

((Seriously, you all knew this question was coming))

The 3D Maneuvering Gear is a technological achievement in itself and yet humans still use cannons and flintlock rifles/pistols? Perhaps it has something to do with the Titans eating the humans to near extinction, but I assumed they would be better equipped to handle the giants.
gpt
In the immediate aftermath of the Titan's appearance and near-extinction of mankind, people were forced to pack up and leave.  All infrastructure was abandoned.  Factories, refineries, natural resources, etc.  The only things humanity has access to are what happened to be located inside the walls when they got there, and what they were capable of building when they arrived.

The first generation had a large number of learned people; Scientists, Engineers, Professors, etc.  They were able to continue advancement during their lifetimes, and develop the advanced 3D-manuevering gear.  Unfortunately, the need to focus on survival prevented mankind from forming any kind of formalized educational system.  All advanced knowledge that couldn't be written down or taught died with that generation.

The 3D gear was tailored to the resources that were most abundant inside the walls, efficient use of high grade steel for melee weapons and extremely efficient gas-powered gear that uses a tiny fraction of what a car or plane would use.  They'll last a long time, are easy enough to make more of with the limited resources available, and the facilities to make more fuel are present.

Mankind is already equipped with the best of many bad options for fighting the Titans.