For the same reason companies shouldn't be able to band together with other companies to not allow raises. They're anti-competitive practices, which eats away at the entire point of having a market, which is for competition to force parties to offer better prices, bid higher amounts, and produce better products/services, which benefits everyone. For example:
- Landlords should not be able to collude to keep rent prices high. They should be forced to compete against each other, either by offering lower rent or better premises and services to tenants. The result is that over time, society gets better and better places to live, that are nicer, updated, and safer, at cheaper prices.
- Healthcare providers shouldn't be allowed to collude to set uniform prices for services. They should have to compete on price, quality of care, or access to treatments, ensuring patients can choose better or more affordable options. The result is that more and more people can afford healthcare services, which themselves become increasingly effective over time.
- Internet service providers shouldn't be able to divide territories or coordinate to prevent competition in specific regions. They should have to compete, driving down prices or increasing service quality for consumers.
- Software companies shouldn't agree to not hire each other's employees to keep wages low. This prevents employees from negotiating higher salaries and better benefits, hurting workforce dynamism and innovation.
Etc.
Capitalism is simply a collection of laws and regulations that blocks all means of profit other than simply offering a better deal or better services. The goal is for those to be the only real ways to profit. The side effect of workers and companies all competing to do this in order to profit is that society benefits by having a ton of innovation to make better and better things, at cheaper and cheaper prices. Which is the central reason why, today, the average person can have a cell phone, a TV, the internet, amazing healthcare treatments, and an almost infinite array of options for clothing, food, entertainment, etc.
Allowing people to profit in ways that disrupt competition gunks up the entire functioning of the market. Maybe you get some short-term benefit, but ultimately you end up with a system that doesn't create nearly as much wealth and prosperity. Because why go through the trouble to create great things for your customers (as a company), your employers (as an employee), or your employees (as an employer) if you can instead benefit by simply banding together with others and colluding, or monopolizing some essential resource, or fixing prices, etc.