it's not what biologists say.
"There are multiple, independent lines of evidence to support the hypothesis that eukaryotes evolved from an endosymbiotic event between an ancient archaean cell and an ancient aerobic bacterium:"
https://organismalbio.biosci.gatech.edu/biodiversity/eukaryotes-and-their-origins/#:~:text=Mitochondria and the origin of eukaryotes&text=The leading hypothesis, called the,an ancient, aerobic bacterial cell.
It only happened once in the entire history of life on earth, and never happened again after that. It's one of the great mysteries. A virtually impossible event, but it happened, or there'd be no higher life on earth. It's why the chance of higher life on other worlds is regarded as extremely low, although there may be a reasonable chance of bacterial life.
That's why scientists are looking for a better explanation than endosymbiosis for the origin of eukaryotes, but they haven't found one and most agree that a single one-off endosymbiosis 'quantum leap' event is the best explanation
(edited)