And the reason knights faded out of existence, despite still being a 1-on-1 match for anything on land, is that they didn't live long enough to get their pointy stick into the bowmen/gunners/pikemen arrayed against them.
There's sort of a constantly swinging pendulum between protection and mobility. Some weapons are very lethal, even through armor. But they have a narrow field of fire (IED's, RPGs, sniper rifles). Others are very difficult to dodge, but have light penetrating power (small arms fire vs vehicles, grenades).
More armor, less mobility. This is something that troops in Iraq found. When they up-armored Humvees (light, mobile vehicles) against IEDs, it slowed them down and made them more vulnerable to RPG fire.
It's just silly to say we never research defense. Personal body armor today is almost sci-fi like in its capabilities compared to even 10 years ago. Combat fatalities per incident are miniscule compared to what they were in previous wars where infantry generally didn't HAVE armor. Flight crews were usually the ones with body armor, and it was just a flak jacket. There's been all kinds of research into anti-mortar systems. There are armored MRAPs specifically designed for use in IED-prone areas.
The problem? All this shit is expensive. And a weapon to defeat it can be cobbled together with the parts available in the average street market. That's asymmetric warfare for you. And it's not a new problem. The civil militias of the Swiss cantons and the Low Countries fended off royal armies sporting fully-armored knights with little more than repurposed farming implements, and tactics and terrain. The locals in Iraq and Afghanistan are pretty good at using tactics and terrain too. Remember, the Taliban captured most of Afghanistan in the 1990's using a fleet of Toyota pickups. Against warlords who had a crapload of abandoned and captured Soviet tanks and artillery. Turns out, tanks and artillery aren't great assets in a country full of mountains and very few developed roads.
For Iraq, especially urban battlegrounds, it was right to focus on up-armoring. For Afghanistan, it's right to focus on goight light and fast. Speed doesn't save you when you're hemmed in with buildings and potentially taking fire from all directions. Armor doesn't save you when it means you're slogging up a mountainside at a snail's pace and even a blind goatherd can keep you in his crosshairs.