@Sandai:
Goku turned Oozaru after he and the other got captured by Pilaf and we're left to die in a special room
No Pilaf in this movie.
in DragonBall (Z) Gohan turned Oozaru and managed to push a persistant Vegeta into a corner until Vegeta ended his transformation which in turn caused Vegeta to admit defeat.
No need for it to play out that way. What's important is Vegeta's defeat, not HOW it happens. There's plenty of other ways for Vegeta to gain the upper hand in the fight, and then subsequently lose it. Having tails and Ozaru for only one fight, when they hold no role or importance anywhere else, would be a waste of time and resources.
If the LOTRs could turn grown men into short trolls & hobbits and what not they can get away with giving a human a tail in DB.
Hobbits and their nature became practically the central focus of the entire story. Excluding Hobbits from LotR would be like not making Goku a Saiyan at all. Simply not including the Saiyan tail would be more akin to not giving the Hobbits hairy feet. A whole 'nother deal on a whole different level of importance.
The tail was something that only really had any significance or importance during early DragonBall. It emphasized Goku's "Monkey King" nature. Once Vegeta showed up, and it was revealed that the tail was a Saiyan trait… it stopped being important. Neither Goku, Gohan, or Vegeta had their tails anymore, and all the attention after that was focused on "the legend of the Super Saiyan." The tails were done with and gone as soon as we found out what they really were about.
The point: So far, tails serve no purpose in the movies. To include them solely for the one tiny possibility remaining would be meaningless. Meaningless and goofy-looking. Better not to include them at all, IMO. If the attention is primarily on Z, then it's better to focus on Super Saiyan instead, and not over-complicate things for the non-fan audiences. Goku isn't the Monkey King in this. He's only a Saiyan, who will eventually become the Super Saiyan. He doesn't need a tail for that.