Well, at least on the ladder, players in the midsection of the ladder and high ladder disagree with Lava Plume being better. From a burst damage perspective and with forcing significant chip in the short-term, Magma Storm is a superior option a lot of the time.
For 1695+ stats:
| Moves
| Earth Power 95.868%
| Stealth Rock 80.343%
| Magma Storm 75.428%
| Taunt 66.298%
| Will-O-Wisp 22.033%
| Lava Plume 20.343%
| Heavy Slam 12.367%
| Protect 10.895%
| Other 16.425%
For 1825+ stat:
| Moves |
| Earth Power 93.147% |
| Stealth Rock 87.383% |
| Magma Storm 78.090% |
| Taunt 49.354% |
| Will-O-Wisp 44.556% |
| Lava Plume 19.637% |
| Heavy Slam 10.453% |
| Other 17.380%
It also isn't necessarily true that fat is the only teamstyle in which Magma Storm is better than Lava Plume. If you happen to hit a mon with Ground-type coverage with Magma Storm, which will happen if one's switch-in to that is Ting-Lu, Moltres with Scorching Sands, or something like Landorus-Therian if you don't have anything else to take a Magma Storm, Heatran's Air Balloon will force the players using them to make more predictable moves as Lando-T or Moltres will want to use U-turn or risk taking more damage or getting Taunted in Moltres's case, which can be punished and is a useful interaction even against more offensive teams.
Additionally, Magma Storm reaches crucial damage thresholds that Lava Plume doesn't, which can mean the difference between forcing a Tera and not, so this is more than just about Lava Plume being more consistent on a turn-by-turn basis at what it does. It is rather reductive to say that Lava Plume is better except against fat given other use cases where Magma Storm is superior even if people who choose to use Magma Storm have to risk Magma Storm being unreliable, but that's a trade-off the players know about.