Math example:
You sell something for $20, and it costs you $5. You have free shipping, which costs you $3. Shop and CC fees are maybe $2, so you're netting $10. Then taxes means you net $6.
So if you sell 10,000 units you've netted $60k. But to make that $60k you need to spend $50k up front to buy the inventory, and maybe $2k more to ship the inventory to you. Then you need a place to put it, so maybe $1k/month.
So when you need to order your next batch of inventory you need, say, $52k. You haven't made that much yet, because your inventory has a 2 month lead time. So you use money from the 60k in "profit" to buy inventory, leaving you no profit.
At that point you haven't made anything. It looks like you're making money, but you aren't. Maybe you dip into your tax reserve fund to buy that inventory, but the fact is that you probably haven't paid yourself much, if anything.
This is the brutal reality of small business math.