First, these industries are dominated by big companies, internet retail is dominated by Amazon, internet software is dominated by Google and socialplatform software is dominated by Facebook.
These companies have power on the market and they can buy a start up if they are scared of the concurrence, the company will not try to make the company grow and then later kill it to be the only one in the market . That's what happened with Picasa when google bought them to avoid rival for google photos.
Moreover big companies can hire the best engineer and so giving less chance for start up to innovate.
That's why there is less and less investment by start-ups.