Because they have the technology, the best professionals, the money, the experience, the research, the logistics. This all means that the small companies couldn't keep up with the pace of growth and quality of the giants. Even if the new business creators would like to be bought by the giants, some would like to keep their businesses long-term, and these big companies would discourage that goal and even make it unattainable. The problem is the monopolies, and stagnation in innovation that would emerge from that. To solve that, the government could implement policies that could protect small businesses, like tax benefits, financial incentives, contests, awards.