Just to note - The strength score listing on heavy armours does one and only one thing if you don't meet it - it reduces your speed by 10 feet. Now speed is valuable, yes, but the difference here is the value of a single action over the course of a whole encounter, generally speaking (as always exception will apply – this is an average general description). Important to note, however, it that it is not a requirement to use the armour, and not meeting it does not impose any other penalties of any sort, other than that single speed reduction.
If you base in Dex, you will use finesse weapons. You will not use thrown weapons unless they are finesse, and you can use ranged weapons more easily, which are almost always the superior choice for things out of your reach anyway; these are not really successful points, considering that you are at no detriment choosing Dex over Strength here, since there is no shortage of suitable finesse weapons, and arguably you are at a disadvantage using strength, since the range of thrown weapons falls far, far short of the range of ranged weapons.
The difference in AC between a 20STR fighter using the best heavy armour, and using all of the same feats and features as a 20Dex fighter using the best light armour is exactly 1 point... and for that one point they have a 20 in what is considered overall to be a far more valuable, useful and more versatile ability score. Dex is, overall, a much more valuable stat than Strength. It's why there exist many ability score setting magic items, even in the lowest grade of items, for Strength, but there are no such equivalent items for Dex - Dex versions of those same items would value up as being one, possibly two, rarity grades higher than the strength ones, if they existed (as would charisma versions, which is why they also do not exist off base... but wisdom and intelligence versions do).
Quick review:
A Dex fighter with defence fighting style and a shield, has an AC of 17 available to them at level 1 in BG3: That is: Leather (11) + 17 Dex (+3), Style (+1), Shield (+2) This becomes 18 if we allow for how rapidly you can acquire Studded/studded equivalent in the game currently.
A Str fighter with defence fighting style and a shield, has an AC of 17 available to them at level 1 in BG3: That is: Ring (14), Style (+1), Shield (+2). This is IF-and-only-IF we hang out on doing the comparrison long enough to GET ring mail, which takes longer than getting Studded/equivalent. If you want Chain, well, last I checked you couldn't stumble upon any chain until after the grove.
If we stretch to chain anyway, it becomes 19, which is still only 1 better than the Dexer at the same point of play (assuming you're not level 4 already - if you are, the Dexer will be 19 as well, and equal).
The Dexer will move to 19 and then 20 by level 6, no other gearing or equipment luck needed.
The Strength fighter will only improve if they find better armour - to 19 with Chain, 20 with Splint and 21 with plate.
It is, ultimately, a small disparity that is more than overshadowed by the greater value of Dex itself over Strength, as an ability score.
The nod to carry weight is probably the most important balancing factor back in Strength's favour, at least as far as video games go. Outside of that, Strength does have some uses out of combat, but realistically not many - general strength checks are moderately rare, and while athletics can be a fairly common roll, stealth, acrobatics, and sleight of hand more common all together, and more generally speaking as well, as are general dexterity checks.
