Say you are not an insider. Most investors in the public markets are not insiders. If a company wanted to be owned by insiders only it would never need to enter the public market. So say you, like most investors are considering stock where you are not an insider. Would you buy stock from an insider? Even if he properly discloses he is an insider. Is he selling because he needs the money to buy a new house or because he knows that something happened and the company is doomed. It does not feel very comfortable does it. So most investors would not feel safe and comfortable buying most stocks. So, good job, you just removed 90% of demand of stocks.
What people do not understand is that the american public markets are very unique, unlikely, and downright amazing phenomenon that is a very important factor in the wealth of the USA and even the world for the second half of the 20th and 21st century. It is amazing that people have the confidence to pay a bunch of money to own a part of a company based only on published information. But that is what makes America rich. It is that ability to quickly move money from where it is not needed to where it is needed and to reward the person sending the not needed money.
Now here is another hypothetical. Say you are insider of a company with a lot of stock in that company. You have just learned that something secret has happened which has doomed your company. Your biggest client said they would stop buying from you or something like that. You want to sell your stock. If insider trading is legal would you inform the public of the bad news? Or would you make sure you sell all of your stock first. Now say the same thing is true but insider trading is banned. You are a big stockholder and you know the SEC is watching you. What do you do? Well you inform the public asap, so that you may sell your stock legally albeit at reduced prices. Which of these outcomes is better for society, and for the well being of the stock market?
I suppose insider trading is hard to enforce. Well actually it is not that hard to enforce for the important people -- the company leadership that have all the important insider information. But there are other crimes that are also hard to enforce but we keep them crimes, because they are terrible, cause a lot of damage and we as a society should do what we can to avoid them. Should we make bicycle theft legal because it is so hard to catch bike thieves? How about date rape?