Accountants are hired under confidentiality agreements. Connecting random third parties to your systems that have "we are allowed to do anything we want at any time" agreements is kind of the exact opposite of that.
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.
Confidentiality agreements aside, the scale isn't the same.
Accountants have tens of clients, online apps have thousands or millions. For an app, the relative benefit from skimming a bit off every client is higher, the relative cost of losing one client who notices is lower.
There is a regulatory body for CPAs, which include things like ethics. There are "generally accepted accounting practices" (GAAP) which are adopted by regulatory bodies like the SEC and IRS. There are laws that are related to all of the above.
Point is, my dealings with my accountant(s) is highly regulated, and there is expectations of confidentiality, trust, and privacy -- with rules and laws to back that up.
Not the case with some random app handling your corporate data. Plus Outlook and other calendars integrate with many other systems, and there are a lot of nasty hacks if you can get someone's email. Lotta risk for negligible gain.