Mostly because if you don't who will? America is run on a capitalist system. If the manufacturer has costs, the consumer has more cost so the manufacturer makes a profit. Now, they shouldn't cost so much, but there is also marketing to consider which includes advertising and packaging. Don't forget shipping costs and sales tax. The last of which brings me back to my point, if you don't pay for your pills, who will? The taxpayer, that's who!
Sorry you need medication for a lifetime. Just be glad it is available to help your condition. You could live someplace where it is not! Buy in bulk if you have insurance. Mail order is an option with most insurance plans that offer the pills in 90 day supplies at a discounted co-pay.
Expenses of manufacturing and selling drugs notwithstanding, many manufacturers are now offering assistance to people who cannot afford their prescriptions. Is it generosity, human compassion? Not likely. Profit it the bottom line with manufacturers, so it is probably a tax write-off for them. Still, look into that option if you cannot afford your meds.