An Animagus is someone within the Wizarding World of Harry Potter who developed the ability to turn into an animal form at will. The concept was introduced early in the series with Professor McGonagall, who possesses the ability to transform into a cat. Other notable Animagus introduced in the series include the Marauders James Potter, Sirius Black, Peter Pettigrew and Hermione's nemesis, Rita Skeeter. Repeatedly throughout the series, someone's status as an Animagus made them particularly difficult to capture or imprison and served as an unlikely secret weapon. But why don't more witches and wizards utilize this ability?
Repeatedly, Animagus are able to hide in plain sight with their powers. Peter Pettigrew was able to spend over a decade in relative safety, posing as the Weasley family's pet rat Scabbers, without alerting them to his true identity. Sirius Black even used his dog form to evade the guards of Azkaban. Later, he -- as well as Rita Skeeter using her bug form -- would prove capable of sneaking into Hogwarts, often considered one of the single most secure buildings in the Wizarding World. Since the ability becomes an innate power upon completion of the original spell, there's no incantation or wand necessary, adding to the stealthy potential of this technique. Magic-users may not even realize animals around them are Animagus, with witches and wizards effectively able to hide in plain sight thanks to their stealthy alternate forms.
Becoming an Animagus is repeatedly shown to be a useful skill for stealth and could have seriously improved the odds for the Order of the Phoenix if their reconnaissance missions could be completed without drawing attention to their spell-crafting. Even though the Ministry of Magic attempts to keep a detailed Registry of Animagus, it's failed to account for people performing the spell and using it to great effect -- such as Rita Skeeter using her minuscule bug form to listen in on conversations and steal secrets for the Daily Prophet. And while there is a spell that can reverse an Animagu back to their natural form -- as used on Pettigrew in Harry Potter and the Prisoner of Azkaban -- the spell-caster would need to be aware that the target was, in fact, an Animagus.
Likely the biggest in-universe reason more characters don't become Animagus is that the process takes a great deal of time, effort and care to perform correctly. Although the enchantment doesn't require a wand, it does need a lengthy process to prepare the user's body. The spell includes keeping a mandrake leaf in your mouth for an entire month, reciting the same incantation at the same time at dusk and dawn, and drinking a specially crafted potion amidst a lightning storm. Performing any of these steps incorrectly would either restart the process or leave the user transformed into a human/animal hybrid. Even for gifted students like James Potter and Sirius Black, the process took almost three years to get right.
But given the various uses of having an Animagus transformation at their disposal, it's surprising more witches and wizards don't attempt to use the enchantment. This is likely the practical reason more characters in the franchise lack those abilities, as having too many Animagus around would introduce too many stealthy characters into a storyline where quietly recovering important information directly impacts the fate of the world multiple times. But regardless, the Animagus enchantment is still one of the franchise's most useful tricks, and it's a shame more characters didn't utilize it.