[2020] Some champions abilities show the stats on every Rank of the Ability. Why do not all champions have this feature?

It's an incomplete feature that only some champs (and only some spells on those champs) have the appropriate data for. You will also notice a few other oddities littered throughout tooltips currently. For example, sometimes you will see icons as scalers and sometimes you won't.
I can't promise exactly when, but all champs WILL get this in the future. I'll provide technical details in a followup comment.

Technical details:

So, the way we used to build tooltips is PRETTY bad. Basically, imagine ALL the text in the tooltip in a single long string. The title, "Mana Cost", and so forth. There are some variable values (like how much mana cost, or how much damage), but for the most part... all the text was clumped together. Importantly, this includes "structural details". And to just outright reveal that ugly truth... here's an old tooltip in its raw form!:

tr "game_spell_tooltip_EzrealMysticShot" = "Mystic Shot (@Level@)[@Hotkey@]@Cost@ Mana@Cooldown@ sec CooldownEzreal fires a bolt of energy, dealing @Effect1Amount@ (+@CharTotalPhysical@) (+@CharAbilityPower2@) physical damage (applies on-hit effects).

Ezreal's cooldowns are reduced by @Effect2Amount@ seconds if Mystic Shot hits a target."

All that stuff is that structural detail. And all of that goes to our localization teams. Which is not an easy mess to sift through if you're not reallllly familiar with the details. This means changing something, especially if that changing something is changing it for every spell or champion in the game, is NOT cheap or easy to do. More pertinent to this particular post, this is what the level up section looks like:

tr "game_spell_levelup_EzrealMysticShot" = "Next Level - click to level-up or press [Ctrl-@Hotkey@].Bonus Damage
Cooldown
Mana Cost@Effect1Amount@ -> @Effect1AmountNL@
@Cooldown@ -> @CooldownNL@
@Cost@ -> @CostNL@"

Tooltips are also no longer built that way. They're now broken apart into a lot more pieces and with a lot more tooling and features around them. This is the core part of Ezreal Q now:

Ezreal fires a bolt of energy, dealing @Damage@ physical damage (applies on-hit effects).

Ezreal's cooldowns are reduced by @CDRefund@ seconds if Mystic Shot hits a target.

And the level up section is broken up into its own very different kind of data that I can't easily represent here.

But updating all the tooltips in the game, extracting all that data, and making sure everything generally works... takes time. If we tried to do everything all at once, our localization teams would reasonably not be able to deal with all of it. So we've been converting things to the new systems over some time now and pausing temporarily whenever there's a lot of work that needs to be done elsewhere that impacts localization.

The vast VAST majority of this work is done. For most champions, especially new champions and reworks that are built into the new system from the start, it takes very little extra to enable the "level up grid" (and icons). Which is why, I imagine, many designers have gone ahead and done that. When we get the remaining to-be-localized elements in though, we will be able to immediately turn this on for ALL champions at once.

If it has the grid, then it isn't typed out manually. There's a system that generates the grid.