The point buy is per the PHB. It's one of the many ways of allocating your attributes (along with 3d6, 4d6 - drop lowest and the "standard set").
You should be able to get to a 17 by allocating points to 15 and then getting a +2 racial bonus (if it applies). Since no implemented race has +2 to INT, the best you can do is a 16.
However, 16 and 17 are both +3, so that shouldn't make any difference.
If you're a min/maxer, it does severely limit the "viable" race/profession combinations. Like, a Tiefling Ranger might be fun, but at best you're going to have a 15 DEX, so +2. Which is not ideal for your primary stat.
(Larian, if you read this, please implement Dispater or Glasya so we have a +1 DEX option for Tiefling!)
From a simplicity to implement and a "fairness" perspective (for multiplayer) I can see why they went with point buy. However, for single players games, there's no reason not to let somebody click "roll again" 400 times if that's what they really want to do.
I'm well aware of the the numerous ways of generating/rolling/allocating points for stats within the 5e PHB.
What I am sating is, the system in which Larian specifically chose for BG3 is GARBAGE. It's not heroic and it makes for a pathetic character.
IN ADDITION, as I stated, no matter what I tried to do with 27 points, I cannot and have NOT BEEN ABLE TO set any stat above 15. I don't care what racial stat bonuses I have for any particular race, the stats that I can manipulate with 27 points, do not go above 15 during character generation.
Hope that clarifies my original post, for anyone that is unable to understand what I stated before.