Because I believe these trends are dependent on the negligible cost of handling blocks, costs which will become significant soon enough if the block-size growth trend continues unabated.
Blocks don't just magically grow in size for no reason.
Blocks get larger because of an increased demand for Bitcoin transactions.
A larger number of bitcoin transactions means more aggregate fee revenue, which means as the block size grows the miners are competing for an expanding market.
Expanding markets attract new entrants into the market, and in the case of Bitcoin mining there is no way for incumbents to exclude competitors who produce valid proof of work.