There's a trade off for the OEMs in producing more power against more emissions there'll be a sweet spot between poke and smoke that they'll juggle to sell more cars to enthusiasts like us or emission conscious company car users who are sensitive to CO2 figures
....This is the bottom line (plus cost factors of course) - Designing cars fit for purpose. Some folks want to redesign the purpose of the OEM car.
As aftermarket expert specialists develop one or several parts, then there must be a knock-on effect - Where does it stop? Even if you start off by swopping the whole engine and transmission to one which delivers much more performance out of the box, you then have to consider how that added power is transmitted and handled by the chassis and suspension etc. It's a very slippery slope even without any polishing.
Also, reliability surely also has to be considered - Cars modified way beyond OEM tend to break more easily. There are plenty of examples on this and other forums.
I've always wondered what percentage of GTI (for example as it's a variation from the standard Golf)) sales are modded by purchasers and whether Volkswagen actually care and cater for this.