There are public platforms for crowd investing of startups, but they require users to be accredited investors due to regulations.
That’s not the real issue, though. The real issue is that good startup teams don’t have any problem raising normal investment capital right now. The only companies who would even want to raise money from the public would be those who couldn’t get normal investment, which are almost exclusively bottom of the barrel companies that have been passed on by many professional investors. For many reasons, not just regulatory, you don’t really want to have thousands or tens of thousands of tiny investors to manage for your startup.
Companies tried to skirt regulations with the ICO craze a few years ago. It was almost universally a complete disaster for investors. A lot of ICO companies got extremely wealthy and a lot of investors discovered that without the regulations and share ownership they don’t really have much of anything.