You remember when bounties were added and there were outcries about 1k gold being ridiculous and too unfair? Then Riot changed the display message to occlude the kill gold and show only the bounty, and suddenly all the crying stopped.
Boots have a similar issue in that showing steelcap value would make it seem far and above the other boots and people would cry for nerfs, but the boots are fine otherwise.
To show the numbers, Riot would have to show numbers for other boots as well in order to drive home that all boots are equally strong in their own strength, but it's harder to quantify how much attack speed or ability haste contributed to your DPS and display that in a number. It's also hard to make people look at a small number for "reduced cc" and realize that sometimes reducing 1s of stun can be more powerful than preventing 1k damage.
Tl,Dr: it's hard for our brains to contextualize the numbers and this change could lead to the boots having to be nerfed to a point where it'd be trolling to buy them.