Imagine you found a cape with 15% Cooldown Bonus at Ascension Level 20 (which is very good). If you now level up and the item would stay at 15% you would probably never find a really good upgrade to this item. At some point you reach Ascension Level 100 and are still using your level 20 cape with bad Leadership value but high Cooldown Bonus. All those levels without finding a nice upgrade!
Some might say, why not start with lower values, e.g. 2% Cooldown Bonus at level 20 and then go up to 15% at max level? Because 2% are barely noticeable, as is an increase of ~1% every 10 Ascension Levels. We always want the stats to be meaningful, even if you just started playing the game we want you to notice the difference of having an item with Cooldown Bonus equipped or not. That’s why players of all levels have the chance to find items which are powerful on their current level. And as Cpt. Morgan said, when you level up you need stronger items with higher values to keep the same percentages.
The problem with raw percentages in general is that they theoretically scale without you having to upgrade anything.
You are Ascension Level 10 and have 25% Physical Resistance.
A warrior hits you for 200 damage but your resistance mitigates this to 150 damage, protecting you from the remaining 50 damage.
Now you have levelled up to Ascension Level 100 and have the same 25% Physical resistance.
A warrior hits you for 2000 damage but your resistance mitigates this to 1500 damage, protecting you from the remaining 500 damage.
As you can see, the same percentage has mitigated 10x more damage on a higher level, which means that 25% at level 100 are way more powerful/valuable than 25% at level 10. This is why you need higher resistance values (as well as values for all other stats) at higher levels to get the same percentages.
I mean it’s the same with damage and health values. Your enemies get stronger as you level up, so you need bigger numbers to be able to survive and beat them.