content: I can speak a bit to the suit of armour, but bullets aren't my what I worked with.  Armour has several competing features. weight, strength, durability, cost,  and flexibility,  all of these things are considered when making armour.  For soldiers, you want something that doesn't weigh a lot, is relatively cheap to produce, and  doesn't prevent them from using their .  weapons.  To make a suit of armour out of titainium wouldn't be cost effective because of the difficulty of working with the product (its hard to cut and hard on the tools you use it with).    Second, its not just preventing the bullet/knife/weapon from hitting you.  Especially with modern armour, the bullet can still kill you based on it just hitting your armour. Impacts hurt, and still break bones even through that material.  Look at the armour for bomb defusors in the military.  That is heavy,  inflexible, and costly.  It does save lives,  the shock wave is generally dissipated by the armour.   It highly limits mobility. And given all that, the armour is still vulnerable, its not 100% proof against all weapons/all bullets.    So, then, why kevlar.  Its pretty good,  it can be resistant to knifes,  it catches most bullets that soldiers come across.  Its easy to mass produce, and  doesn't need custom sizing like a suit of armour typically needs. now, yes you can make something that is damn near bullet proof.    I recall  https://www.youtube.com/watch?v=24n77GgRtrw  which is an example, but anything that might send a shock wave through it would be particular devastating.