In the supernatural world of Bleach, the biggest question is not what a person does in life, but what happens to their soul when they die. The default is for a black-robed Soul Reaper to perform the konso ritual and escort that soul to the peaceful Soul Society, but not all souls take this route.
If a deceased person had attachments, resentment or other issues in life, then their soul remains chained to the earth, and this chain will tear a round hole in their chest. The soul will then mutate into a massive beast with a hole for a heart and a skull mask: a Hollow. These spiritual entities can take any variety of forms and can rampage across the land, using their abilities to trap and eat souls. This might sound like fun, but in reality, a Hollow's life is one horror after another, even for the strong ones.
Hollows' Strength Is Empty
Hollows appear right away in Bleach and most of them are made up of the soul of a single person. But they don't look like the humans they used to be; each Hollow looks different. They may be humanoid, have wings, resemble a frog, have tentacles or extra arms, and so on. Some even resemble centipedes (a common horror visual trope) or even have unique abilities that are a bit like zanpakuto shikai. For example, the Hollow known as Shrieker could emit powerful sound waves to stun its enemies, and it could also launch a number of leech-like entities at its target to trap them. Meanwhile, the fearsome Hollow known as Grand Fisher could use a lure at the end of a long strand to confuse and disorient its enemies, fooling them into lowering their guard before attacking.
At first, it seems like turning into a Hollow could be a life of freedom, since they are clearly comfortable with their bodies and abilities. Some of them seem to definitely have fun in their new lifestyle, such as Shrieker. It looks like a form of supernatural escapism, such as a person morphing into a werewolf so they can run free as nature intended. But there is nothing glamorous about this.
Hollows have such strength and abilities because they are hungry and nothing can sate that appetite no matter how many souls they eat. It's a futile quest, and a Hollow's strength is meaningless in the face of it. Being strong means nothing if the end goal keeps moving as the Hollow approaches it. And as Acidwire (the Hollow of Orihime's brother Sora) showed, many Hollows are in agony over their hunger and lack of resolution to their real-life problems. None of Acidwire's strength could fix its problems, and only Ichigo's intervention as a substitute Soul Reaper could make things right.
The Hollow World Is Harsh & Dangerous
It should be noted that while a Hollow may be empty on the inside, it is not alone. Many Hollows roam the earth, competing with one another to hunt human souls. To put it mildly, Hollows are not pack-oriented or predisposed to make friends -- even out of necessity. Instead, a Hollow might fight other Hollows over good hunting grounds, and Hueco Mundo (the Hollow world) is even more dangerous. A Hollow may travel to Hueco Mundo to escape the wrath of roaming Soul Reapers, but many cannibalistic Hollows reside there, so a Hollow who opens a garganta to go there is really just trading one enemy for another. Between Soul Reapers and cannibal Hollows, a typical Hollow has a lot of enemies and must look over its shoulder constantly. That's a stressful lifestyle, to say the least.
Cannibal Hollows can get the edge by eating their own kind and forming a Gillian-class Menos Grande with other Hollows, and a certain Hollow can take control of the amalgam and have its autonomy restored. This is the next step in Hollow evolution. That Gillian will grow stronger, maybe even evolve into an Adjuchas-class one day. The problem is that a Gillian's hunger is even more acute and if a Gillian or Adjuchas fails to sustain itself on other Hollows, it will regress into a mindless Gillian.
Finally, there is no substantial end goal for any Hollow. These spirits hunt and fight just to survive, but there is no salvation or paradise waiting for them. As the Arrancar Nnoitora Gilga once noted, there is no salvation for Hollows or Arrancars, and not even the mighty Sosuke Aizen can change that fact. A Hollow's life is indeed hollow, and it plays the dog-eat-dog game simply because it fears destruction. That's no way to live, and even the powerful Espada, who command minions of their own, aren't really going anywhere fast.