Because if you trade the nearest month's Euro futures instead of EUR/USD, volume is available as well. The correlation between price movements in the EUR/USD and price movements in the nearest Euro future is about 99.9%, so the question you need to ask yourself is whether you want to trade the same thing
with volume available, rather than
without it being available. Not a very difficult decision, really.
People trading EUR/USD against counterparty market-makers in the spot forex arena are those without enough working capital to trade futures. If you have that, there's no advantage in trading the EUR/USD.
And now we'll doubtless see whether anyone finds this view "contentious".