Songwriters signed to contracts, producers and arrangers signed to Contracts, Artists and the musicians all signed to contracts , so I would assume they were left to make music and then the powers that be decided which tracks made it to singles then to the lp tracks. Does seem a lot of great tracks never made it to record but it would be cheaper to can any tracks that the production control thought not suitable for release than to take a punt and put them into production and incur the cost of pressing the records , distribution and promotion only to be left in the deletion bins.
And we probably do not appreciate how many records were be released in Detroit on umpteen labels alone every week during the Motown golden years, the record buyers had a treasure trove of soul music to buy but only so much money to spend.