I don't understand your analysis on this comparing to Dex.
Simple. Some players do not want to waste time boosting Con since it mainly helps for hit points. So instead of boosting Con, they boost Dex and get a much bigger bang for their buck.
For example, the 12 Con wizard in my example above who eventually gets the same number of hit points by taking Tough and instead of boosting Con 4 times, he boosts Dex 4 times. He has the same number of hit points eventually, a lot more hit points at lower levels starting at level 4, +1 to AC/Init/Dex/Dex saves/Stealth at level 12, +2 to AC/Init/Dex/Dex saves/Stealth at level 19, and it cost him 7 levels of his Int being 1 less, and 8 levels where his Con saves/checks were 1 less, and 5 levels where his Con saves/checks were 2 less (assuming an even starting Con, that's about 1% of all hits on him where the Con difference matters).
AC is attacked all of the time (about 3% where the Dex boost matters for AC alone, not including Init, Stealth, Dex saves, etc.). Having more hit points at level 4 onwards and more AC at levels 12 and 19 might be very worth it.
So the tradeoff is +1 to Con Saves and +1 to Con checks and +1 to hit points, vs +0 to Con saves and +0 to Con checks and +2 to hit points. Seems like a fair trade to me. Definitely not a no-brainer, and also not comparable to +4 to an ability.
Appearances can be deceiving.
As per my example above where the Wizard goes from 17 hit points at level 3 to 30 hit points at level 4.
8 bonus hit points in a single level and eventually 40 hit points, a total of 17 levels where he gains significantly, vs. losing +1 to two other ability score mods for 7 levels (the other 10 levels do not matter since a 19 ability score is no different than an 18 ability score, etc.).
Or say he waits until level 12. Then he gets bonus hit points (starting at 24) for 9 levels vs. losing +1 mod to two other ability scores for 3 levels.
Or say it is a human doing it at level one. Bonus hit points for 20 levels plus a skill vs. losing +1 to four other ability scores for either 7 or 10 levels depending on if they are even or odd. Here, his main ability score does not even get touched.
The more I think about it, +1 per level is not enough. But +2 per level, although not a no brainer, is a very strong candidate for a feat for nearly any class/race. Hill Dwarf fighters can go nuts with this and have 284 hit points at level 20 (60 more hit points than other fighters who do not take this feat, that's >25% more damage he can handle due to this feat and his racial ability). It would take the 16 HD dragon in the Starter Set about 27 rounds to take down this dwarf, it would take the dwarf about 3 rounds if that to take out the dragon, a 20HD dragon might take upwards of 5 rounds. High level fighters are nasty in 5E.