For the extremely obvious reason that an insurance company would be out of its tiny mind to offer a policy where YOU can decide when the "accident" happens. Or else it would be easy to bankrupt the company - every intending suicide would want to take out a policy before doing the deed, and get a big payout for whoever they leave behind. If they allowed that, they'd soon go bust!
In fact they generally will cover suicides, but only after you've had the policy for a reasonable time which will be stated in the policy. Maybe a couple of years. Then if you commit suicide, it won't have been something you were planning on when you signed up for the insurance.
The intention of insurance is to cover you in case of an expensive accident, and insurance won't pay out if the company can prove that you deliberately caused it. For example, deliberately crashing your car and then claiming - that's also the crime of insurance fraud. Similarly, so is killing someone so you can benefit from their life insurance.
Insurance is a bet by the company that what you want to insure against won't happen, and they set the price at a level where (hopefully) they will make enough money be able to pay out on the occasions when it does and still make a profit. It's not going to work if they pay out for things that aren't accidents. That will throw all the calculations out. It's all based on statistics of how likely it is that things will happen and they employ actuaries, who are experts in statistics. Which is why you get asked all kinds of questions when you take out insurance. This is so they can fit your facts into the actuarial model and come up with the best price they can. For example, young drivers always get charged more for car insurance because we know they have the most accidents.
Life insurance is a bit different because we all have a 100% probability of dying, so the bet is WHEN this will happen. Whole life insurance always pays out so the price is set so that most people will end up paying far more in than they ever get out. (So you can ask yourself whether it's ever worth having. Why not just invest the money instead?) Term insurance is cheaper because the bet is that you won't die in the next X years. If it covered suicide, the price would have to be FAR more.