Profitability is just a proxy for whether someone is legitimately running a business, or just trying to save money on their hobby. Businesses can deduct expenses, hobbies cannot.
So if you are running an etsy store or an engineering company and buy a 3d printer to make parts, the cost of that 3d printer is subtracted from revenue for tax purposes. If your “business” is actually a hobby, it’s not legally a business expense and therefore it’s not deductible