Why are the financial statements submitted to the government for tax purposes not always the same as the financial statements sent to investors?

I understand that businesses want to make their numbers look good to investors, while also paying as few taxes as possible to maximize profits. However, the IRS and the GAAP both have different rules when it comes to recording financial information. This results in businesses having two different sets of accounting records; duplicating work in this way seems inefficient and potentially unethical. Why are the IRS and GAAP holding accountants to different reporting rules?