As others said, accountants are hired under confidentiality agreements. What that means is, should said accountant breach my trust, they're an easy target to hit with a lawsuit. The possibility discourages scammy behavior and establishes some baseline trust.
Big corporations are also good targets for lawsuits - they may not be easy, but they are stationary, and if you have a good case, chances are many other people have one too. This is, again, establishing some baseline of trust, even in absence of a proper business contact.
Random SaaS / fresh startups? They're a highly-mobile targets. There's a good chance they may close shop and disappear overnight. There is no baseline of trust there, and much more thorough due diligence is required.
Big corporations are also good targets for lawsuits - they may not be easy, but they are stationary, and if you have a good case, chances are many other people have one too. This is, again, establishing some baseline of trust, even in absence of a proper business contact.
Random SaaS / fresh startups? They're a highly-mobile targets. There's a good chance they may close shop and disappear overnight. There is no baseline of trust there, and much more thorough due diligence is required.