Although the majority of flights I book come with a receipt automatically emailed to my inbox, there are still several cases where I am provided with an itinerary but no receipt. Why would any company do this? Receipts are frequently needed for reimbursements, and there doesn't seem to be an advantage of not sending one.
One theory is that the many layers of affiliates make tracking the receipt difficult (e.g., a purchase goes through through Kayak then through Travelocity to United). But even purchases direct from some airlines like JetBlue send you through hoops to just get a simple receipt. If you call and ask, they are cagey with their response, or offer ridiculous excuses like "people don't need receipts that often".
Is there some nefarious reason they want to make it hard? Or is it just incompetence?