Yes and no. If the answer is:
- learn how to be a sysadmin + programmer
- build your own chip + PCB + computer
- compile your own kernel before self-hosting it
It would be quite unrealistic and hardcore to reach that kind of independence.
Not impossible but not to everybody’s capability/resources.
Even if yes, if you don’t do that 24/7/365, there is no way to tell.
Some companies are just more trustworthy than others.
In practice, you also don’t control what’s running on your computer given updates on the fly or random bits flipping in your RAM.
You could have some audits, controls and privacy controls that randomly pop out to maybe tighten the bolts but yes, realistically you can always keep things clean during inspections or use dirty tricks like that.
Now, it would also mean that anything you buy could be malicious.
Plot twist, you do buy 99% of your stuff from Asia.
Since companies there can always install some small hardware piece and spy on you without you ever knowing until some expert finds it but again, it might not appear on another batch so hard to really pinpoint the issue.
Does it mean that you should not use any tech ever and just go live in the forest?
I guess that it is an unrealistic extreme that nobody is ready to accept. 
Hence why @kuebic said that you’re probably not worth the time/effort for those companies to do something as random as snooping on a random person in a huge database list.
Also, if somebody finds out that this is an actual practice there, the entire company could go down because of laws/regulations. For what? Snooping on your summer holidays photos? Probably not worth burning down an entire business for. But in theory very much feasible yes.
As a reminder, some companies are built around entire street-cred/trust.
If this turns poorly with a scandal of such ill practices, people will move to a competitor
Just like if Signal just sells your data to the first guy showing up: nobody will recommend it ever after. Was it worth selling your data for 5$? Probably not in the long run, hence why doing that? Not worth it.
Also, GDPR/other regulations are quite expensive to have the luxury of snooping around on photos.
If the FBI knocks at Hetzner’s doors saying that a user 123456 is probably a pedophile or alike, then it’s worth investigating 5min and double-checking if it’s true by inspecting the content of the VPS of that user.
Or if you’re Facebook, you’re probably above the law and don’t really care about a 50M$ fine.
Just don’t mess up with an ex that is a developer at a billion company and you should be good.