lets run the numbers in a scenario:
a pool of 20% hashrate
gets ~29blocks = 181.25btc a day roughly
the fee's of that day is usually 0.013 a block = 0.377 a day
now imagine over a whole day
would you prefer the pool manager manually selects transactions, does all the checks and sigops on thousands of tx a block template collation causing a delay in blockhashing start times. meaning that instead of 181.25btc average daily coin... it may get 1 less block= 175btc due to messing around with tx burden/selection just for 0.377. meaning 175.377
which would you care more for
181.25 or 175.377
as you can see. saving time by doing less block collating means better chance of earning an extra block per day which reaps better income then doing the transaction management/selection
This logic is useful only if another pool gets a target hash in the same time, which happens rather rarely.
In most cases that competition for milliseconds between pools doesn't exist. It's not like all pools find the block all the time exactly at 11:23:56.777 and being the quickest really matters.
So normally the pool can easily grab both the block and the fees. Sometimes it may not be able do that, but it doesn't know when is that.
Maybe I'm missing something, but really, if you would be right, all miners would just mine empty blocks.