Non-qualified stock options are taxed on exercise but if I just bought say a standard exchange traded call option and exercised it, I would not be taxed until I actually sold the stock.
You are comparing apples and oranges. The employee stock options are part of your income. The level of your income is the value of what you got when you exercised the option. If you chose not to sell, then it is your risk.
Because congress realized that the options have value and recognized the NSOs as employee compensation. Thus when the money is available to the employee (even if just converted to stock), they become subject to fica/mc, withholding etc.
The general rule is that anything you receive from your employer is taxable income. To wait until you sell the stock would be impossible for the employer to track.