You are thinking about it this way: "The longer I wait to exericse, the more knowledge and information I'll have, thus the more confidence I can have that I'll be able to sell at a profit, minimizing risk. If I exercise early and still have to wait, there may never be a chance I can sell at a profit, and I'll have lost the money I paid to exercise and any tax I had to pay when I exercised."
All of that is true. But if you exercise early:
The fair market value of the stock will probably be lower, so you may pay less income tax when you exercise. (This depends on your tax situation. Currently, ISO exercises affect your AMT.)
If the company goes through a phase where the value is unusually high, you'll be able to sell and still get the tax benefits because you exercised earlier.
You avoid the nightmare scenario where you leave the company (voluntarily or not) and can't afford to exercise your options because of the tax implications.
In many realistic cases, exercising earlier means less risk.
Imagine if you're working at a company that is privately held and you expect to be there for another year or so. You are very optimistic about the company, but not sure when it will IPO or get acquired and that may be several years off. The fair market value of the stock is low now, but may be much higher in a year.
In this case, it makes a lot of sense to exercise now. The cost is low because the fair market value is low so it won't result in a huge tax bill. And then when you leave in a year, you won't have to choose between forfeiting your options or borrowing money to pay the much higher taxes due to exercise them then.