Miners pool their hashpower together to reduce payout variance. The larger the pool, the lower the variance, which unfortunately incentivizes centralization. Today, over 50% of the total hashrate is directed to just 3 mining pools. How much of a problem this really is is of course debatable.
However, couldn't miners reduce variance even further by frequently switching pools, say multiple times per minute, in order to get a share of most mined blocks? Shouldn't this incentivize decentralization? Why aren't miners doing this?