It would be a (financially) bad choice for corpo. And answer why, is the same in many similar questions 'why can't corpo do this-and-that'.
Here it is (google as an example only, simplified) the answer:
* Put two googles side by side. competing.
* one is a current one, earning money from ads on you being product, and keeping it under the radar (although in fine print etc etc)
* second is the one devised: premium accounts plus free w/ads
* wait 5 years and observe by market efficiency evolution which of the (competing) companies wins. The first one. In this specific scenario, the second loses because time and money spent on 'premium accounts' will not be compensated by revenues from it. While at the same time, the first google, spending this capital difference purely on the ads department will make its ads department (here 100% of it) much better than ads part in second company. hereby winning on the market.
In general (as the google example is SIMPLIFIED, due to the alphabet scale. Please don't use google apps FOR WORK (emphasis mine ;) as an counter example - it's B2B product :) it is because: we, humans, don't like companies who use fine print etc, yet those companies win case by case with the 'moral, pure pricing and fine ethics' companies BECAUSE OF BIASES AND ERRORS (+) OF HUMAN BRAINS during choice making shopping, exploited day by day.
Thats why.
(+) read stupidity also known why-I-buy-overpriced-sweets-at-checkout ;)