It is called hiring people. You don't have to give everyone and his mom an ownership stake in a business. When you go into a partnership with another person you are basically marrying that person, their spouse and any family members they have. If your partner unexpectedly dies, now you have one of their family members as your new partner. Your partner decides to get a divorce. Well now your business is being torn apart by divorce lawyers.
Joining a partnership only increases risks involved with starting a business. The only exception to this rule is if you are joining a firm for attorneys, accountants, or doctors.
Well a partnership makes it a lot easier to raise capital. A small business can be quite a capital sink. Unless you have the hundreds of thousands of dollars you can personally lose it can be very difficult to expand without either a partnership, LLC, or corporate structure. Given the success rate of a small biz simply having $100K+ isn't sufficient the startup capital has to be a small enough % of one's net worth that a complete loss (which is probable even likely) won't be damaging.
There aren't that many people with millions of spare dollars just laying around who suddenly also happen to come up with a great idea, and have the entrepreneurial spirit so they can startup a brand new company hire talent for pure salary only, and keep all ownership and management decisions to themselves.
There is a reason why "joint venture" structures (LLC, Corporations, Partnerships) exist. Still your larger point is a good one. Know your partners completely and don't take on more than you absolutely need. The more owners there are the more complicated it gets.