Yes, because the checkout page gets revenue ⇒ the bottom line.

The points you say are the things Valve is doing extra to provide value to the customer.
The point I am saying, is the first thing that anyone working for money needs to care about.

steampowered checkout page:

• loads in a reasonable time
• has the same colour theme as the rest of the website

Epic checkout page:

• even the in-app checkout page is slower than the Steam web checkout page
  • is slow not only before loading but also after clicking the 'place order' button
• White colour on dark theme website/app