Just for my understanding, why can't outsource company do everything from A-to-Z without my involvement?
Many, many reasons.
The first being that there are so many untrustworthy SEO companies out there. I've seen far too many people hire them with no idea of what they're doing for them, and then get taken advantage of. You must be an educated consumer of SEO if you outsource it. After awhile, obviously, once you see that your traffic and conversions (and profits) have increased, you can probably let them have at it.
In addition, you know your business, not them. They will need you to provide lots of information about what it is you do and offer. You'll need to review anything that they write to ensure it's correct and providing the appropriate voice for your business.
Or on the other side, why an in-house SEO will should be better than outside company?
Because presumably they'll be focused on you and your company and that alone. They'll get a chance to understand what it is you are all about, your corporate culture, your products or services, etc. While they will probably need some education about all that, in the end, they're there to serve you and you shouldn't have to worry about them screwing you over.
Do you think SEO professionals are usually willing\want\looking for an in-house jobs?
No idea. I guess it depends on how much you're willing to pay them and whether or not your company seems like a good fit for them.
And in general, from your experience, an e-commerce website that wants to reach more potential buyers, really need an heavy SEO activity, or basic SEO of an hour a day should do the job?
An hour a day? You'd need that (and more) just to focus on social media activity to promote your website, company and the content that presumably someone will be writing.
Bottom line is that SEO is no longer something that you can just do by sticking keywords somewhere. It's an all-encompassing online marketing and branding of your business.