Why do stores with high revenue still lose money?

Because revenue is a measure of activity and not of profit. A store can grow revenue indefinitely by buying it, and many do without noticing they are paying more than the customer is worth.

In more detail

The number that decides it is contribution per order after advertising, payment fees, shipping and returns. Track that one figure by product line and the loss-makers are usually obvious and usually the best-selling ones.

What to do about it

Selling online is not passive income. It is a business with stock, customers and a support inbox, and anybody describing it as passive is selling a course.

The number that decides whether a store works is not revenue, it is what is left after the cost of the goods and the cost of getting the customer. Plenty of stores with impressive revenue make nothing.

Repeat customers are the whole game. Winning a customer costs money; selling to them again costs almost nothing.

The mobile app angle

This is where an app earns its place — it exists to make the second and third purchase easier, which is where the profit is.