Because the income is spread to a very few people - those lucky enough to own a pizza or coffee store along popular tourist routes. Everyone else doesn't profit at all from the tourists or makes only pittances.
That is not enough to sustain a healthy city, which led to most of Venice being left for grabs for AirBnBs - good luck finding regular Venetians actually living in their city.
Responsible tourists not only buy food and souvenirs and tours, they also hire carpenters and assistant key account managers, as well as purchase sheet metal, plastic pylons and diesel engines to support local jobs.
Saying that only very few people profit from the tourists is like saying only very few people profit from the factory behind the city.
You are totally right that only few people profit but this is the case with everything unfortunately.
Other types of tourists spend much more time in the city, they sleep in hotels, they also visit more than just the Piazza San Marco (and thus, distribute traffic flow much more evenly).
That is not enough to sustain a healthy city, which led to most of Venice being left for grabs for AirBnBs - good luck finding regular Venetians actually living in their city.