Let's say my site lets people purchase news articles for 1 cent. I get 4 readers in Chicago who each purchase 1 article per week. That's 208 transactions for a total of $2.08.
That's over the threshold in Chicago's state, Illinois, for creating an "economic nexus". Their threshold is $100k sales per year or 200 transactions per year. So now I've got to register with Illinois and do regular tax filing. I may also have to collect more data than I want on my readers there, such as address information, to figure out the right rate (rates vary by location within most states).
23 other states also have that annoying "or 200 transactions" condition on their economic nexus laws.
Compare this to if I instead only offer my news articles through an aggregator. Now when those 4 readers in Chicago buy my articles they are doing business with the aggregator. I don't have to deal with any Illinois taxes. Or any of those other 23 states' taxes.
That's the aggregator's problem. That's fine because the aggregator presumably has not just my 4 readers in Illinois but also plenty of readers of the other content they aggregate, making their business in the state large enough that the costs of dealing with taxes are comparatively small.