Well, okay- here's a little bit of math.
There are 4 kinds of runes- marks, seals, glyphs, and quints.
We'll assume that I never want to split up my runes, so each category is all the same kind (9 attack speed marks, 9 hybrid pen marks, &c.)
Just for ADC marks, I could use Attack speed (scaling or flat), attack damage(scaling or flat), crit chance, crit damage, physical pen, or hybrid pen (If I'm Miss Fortune or another hybrid damage carry). That's 8 options
ADC again, but this time seals- I could use armor to mitigate the enemy adc to help with trades (Scaling or flat), health (same reason, but ubiquitous)(flat, scaling, and % don't get % if you're not tanking.), or health regen to stay in lane after trades (flat or scaling). That's 6 options.
At this point, even ignoring glyphs and quints altogether, ADC rune pages have (86=) 48 options of combinations for seals and marks. Multiply that again by glyphs- flat/scaling mana, flat/scaling mana regen, flat/scaling magic resist (486=288 combinations)
And again by possible quints- Armor/scaling, AD/scaling, armorPen, Attack speed/scaling, Health/scaling, Life steal, hybrid pen. Another 11 options, shooting up the possible combinations for ADC rune pages to a whopping (288*11=) 3168 combinations! That doesn't include any runes that only a tank or apc would want, nor does it include some more niche things, like gp/10, movespeed quints, %max hp seals/quints, crit damage seals/glyphs, crit chance seals/glyphs. It also doesn't include some of the cheeky things like putting in 1 crit chance mark and similar things.
Even if you want to cut it in half to get rid of scaling or flat, that's still over a thousand different pages just for ADC's. Take out hybrid pen too! It's still over a thousand.
20 is pretty limiting when you've got some context.