Why are stat boosts reduced to +1? I thought the reason why they were +2 in 3e was because that was equivalent to a +1 ability bonus (since ability scores and ability bonuses scale 1:2).
Because WotC is prioritzing the old school roll your attributes or use an array with odd values in it approach to ability scores where a +1 added to an odd score will result in a +1 bump. The result too is that humans are very nearly broken in point buy as the cost jumps are always on the evens so +1 to every stat amounts to typically +5 to your total ability modifiers because you only need assign an 11 for a +1, 13 for a +2 or 15 for a +3 (and under arrays and point buy the highest you can assign is a 15 so that +2 is still only bumping you to a 17 (+3).
That doesn’t make sense. 5e uses the same bonuses as 3e.
https://roll20.net/compendium/dnd5e/Ability%20Scores#content
Same stat bonuses, yes. But different point buy costs for attributes.
Basically, in 3e using Point Buy you ALWAYS bought as many stats as you could up to even numbers to get the most bonus for your points (i.e. get a 10 or a 12, not an 11) and all the racial mods were in even increments. You could also buy your stats up as high as 18 before racial modifiers (though in my experience 16 was more typical in all but the most high powered games.
By contrast, those +1s in 5e meant that it was sometimes better to buy a stat up to an odd value because the +1 would push it to an even value (i.e. buy up to 15 +1 to make it 16). You also could not use point buy in 5e to buy a starting stat higher than a 15 (+2).
And the trick with default human under 27 point point buy is you get assign a 15, 13, 13, 13, 11, 8 and end up with a 16 (+3), 14 (+2), 14 (+2), 14 (+2), 12 (+1), 9 (-1)... so a 16 in your prime attribute, at least a 14 in each of the primary save stats (if your class' prime stat isn't Dex, Con or Wis alread) and a +1 in something else to boot. That also means at least +2 hp/level, +2 to AC, +2 to Perception and +2 to hit and damage with light/ranged weapons... AND you can be any class you want with these.
Option two for default human using point buy is 15, 15, 15, 8, 8, 8... which works really well if Dex, Con or Wis is your class' key stat since now you've got a at least a +3 to all the main saves, +3 hp/level, +3 to perception and +3 to AC and to hit and damage with light and ranged weapons. Sure they're crazy vulnerable in that they're not strong, intelligent or charming... but some rogue, ranger, monk or a light weapon focused barbarian wouldn't be TOO inconvenienced by that.
By contrast for a nonhuman with a +2 and +1, your best array in point buy is 15 (to go with the +1), 14 (to go with with the +2), 12, 12, 10, 8... which results in a +3, +3, +1, +1, +0, -1... and you better pick a class where its key attribute aligns with one of your racial bonuses.
+6 and the ability to leverage the point savings of odd scores is VERY useful. It can get downright broken if you end up with four or more odd stats using random generation. +1 to all gets really tempting when you rolled a 17, 15, 15, 13, 11, 10 on your 4d6 drop lowest.