People do it with all pets that have any sort of serious requirements, from parrots, to large lizards or snakes, to tortoises, to horses, pigs and even dogs.

Large monitor lizards are probably fairly rare overall, in the number of types of pets people get. The largest number of pets are always going to be more traditional, dogs, cats, hamsters, etc. In reptiles, the smaller lizards are most common, with small snakes, then large snakes before anyone gets to the larger lizards.

People get horses without realizing they need regular exercise, space, massive amount of food, vet care, hoof care, training... and they neglect and ruin them when they aren't new anymore. People get large dogs and forget it will need exercise and space and training and grooming and tons of food and vet care... and they take them to a shelter when it's obvious they're not all fun and games.

A lot of people get monitors without realizing how much space, food and time they are going to take. Some people research and then promptly think that they can still do things differently, i.e. just let them roam the house instead of a properly heated large cage. Or they fail to research what a full grown lizard requires, versus the baby they purchase. Many of those lizards die.

A few people do research and plan and do things right. There's quite a few folks who do right by their monitor lizards. But you're not going to hear about 90% of those successful owners. You WILL hear about any owner who dumps their lizard, who lets it starve or die, puts it on craigslist, etc. Don't let the louder voice convince you that the quiet keepers don't exist though.