They do it because they can. Ultimately YOU have to decide if the risk of losing them as a client is worth the trouble/hassle of trying to force them to pay on time. Charge them more to account for the COST of your money they are using while they wait to pay you. That is the simple answer.
To a large company 15 day-invoice is like a on-the-spot payment.
To prevent outright theft and to thwart schemes, fake invoicing and the like, your invoice has to pass thru several in-boxes manned mostly by lazy fat women before payment is sent.
As an accountant who years ago audited many large companies, I can assure that is the norm. Large companies who have many (much smaller) suppliers literally have them by the short hairs. They will take the discount even though it usually will be 90 days before the check is cut. Not much you can do about it unless you provide a service that is scarce.
It is not done out of malice, just the way business is conducted.
Partner in a small law firm here. We face the same situation with a few of our larger clients and one thing we have done is basically build late payments into our invoicing. So we bill them one amount, and then tell them if you pay by x date we'll knock ten percent or 15 percent off of your invoice. Seems to work pretty well with some of our clients.
Night Runner states something that is became obvious to me after two years as a small, one-person, business owner.
Take some advise. Use a 30-day net payment for invoices, and assume you'll paid in 45 days. If your invoice is sent out a little late --after the other companies billing cutoff date-- add 30 more days to expect payment.
Big companies don't process and pay invoices as they receive them. Their accounting departments process different paperwork (accounts receivable, accounts payable, etc.) at different time during the month. That's way an invoice that is late 1 day, based on their internal calendar, might not get processed for an additional 30 days.