A big part of damage creep isn't just the raw numbers, it's the items people buy early on.
I'll just give a couple examples.
Lets say you're playing midlane mage.
Back in Season 3, this is more or less what your early game itemization would look like.
The most common item for your first back was 
.
Other common pickups were 
or maybe even 
if you needed it.
Your first item would typically be 
At 20 minutes, some typical builds would be:
If you'll notice, most of the builds back then were based on Mana and Defense, not raw murderizing potential.
The highest raw damage you'd ever see is a
rush on a 
, and it'd be at the cost of total reliance on the jungler gifting blue buff.
Nowadays, your first back is just going to be 
or 
Your first major item is going to be 
or 
if you're a hipster.
Your other partial items are just going to be 
, maybe a 
if you're feeling cheeky.
Other builds haven't fared much better.
Remember when 
was the routine rush item for basically every ADC?
Remember when 
rushed 
?
Remember when the only sources of flat armor pen in the game were
and 
?
Remember when 
cost 2300 for 35% armor pen?
Remember when flat pen had interactions with % pen that made it super inefficient to stack both, meaning a champion had to choose whether they wanted to burst squishies or tanks instead of just building to murder everything?
