Transactions are not needed for collusion or price-fixing. Those happen through regular communication channels. Economic coordination happens through price signals and commercial exchange. The more friction is attached to commercial exchange, the less people trade, and the fewer opportunities to coordinate for the benefit of all involved parties can be exploited.

In other words, taxing transactions reduces division of labor, which is one of the main sources of productivity:

https://en.wikipedia.org/wiki/Division_of_labour