I don't think the math really supports this and I don't think Con is behind dex, I think it is also behind Wisdom and if it wasn't for being a common save it would be dead last IMO.
hit points are easy to come by in 5E and healing is plentiful. The biggest weakness of Constitution is that is not used for any skills.
Most of my characters who do point buy or standard array play with a 10 or 12 Constitution. The only point buy character I;ve played that started with higher than a 12 is a Rune Knight and only because he needed it for DC against his rune knight powers. I would use a higher constitution on a Barbarian too, but I have never played one myself. Everything else I can always find a better stat to put points towards. If you are rolling abilities, like I do for a lot of my games, then it is a different story.
It is not about what you think the math supports and which characters you played. A +1 bonus is just 5%, a 1 in 20 chance that it matters. So having 1 point more dex at level 5 means you prevent about 10 to 14 damage (firebolt or fire ball) in 1 of 20 cases. 5 more hp will help you more often... As a wizard, 5 more hp going up from 10 are about 23% more hp. And more importantly raises your hp so high that you might survive an unsaved fireball about 40% of the times instead of only about 10% of the times.
If you go up from 12 to 14, it is still an 18% increase in hp, and you will survive a fireball with an unsuccessful save with about 75% chance.
Yes, constitution is not used for any skill by default (but mightcome into play if you are doing athletics for a longer duration), but I take my chances with a bit higher HP and better concentration saves most of the time. If you value +1 dex for AC and initiative higher, that is ok. But con and HP are very valuable.