They also tend to be redesigned every 3-7 years because they're huge multimillion dollar projects due to their size and complexity, hence they can't just follow current fads. They need to follow a design which will still be approriate in a decades time.
Startups just do it. And are able to use whatever tools/people necessary to make it happen.
Usual corporate sites want to tell everything about themselves. That's why they opt to display it in such fashion: long prose that don't make sense. That's why it subconsciously looks 'ugly' because you don't even know what you're looking at. The content of corporate websites aren't intended for capturing people, majority of corporate websites are intended to inform - think of it like a company brochure.
Decision makers that want this "web presence" are 1) not educated enough to realize the difference between print and digital media, and 2) not educated enough about the cost of web development. That's why corporate websites can be likened to a huge company brochure that doesn't make sense when translated into the web. To top it off with the already mentioned comments about "approvals", instead of working with designers to make sure the website looks highly presentable and understandable, designers are constantly overridden with the 'preferences' of these decision makers.
In a startup, things are different. People work closely together to bring the organization into a higher level. The key decision makers know that they only serve as a visionary to a team of people, and they know they have to work closely with highly talented people in order to bring the organization to life. This setup results into a deeper level of communication that allows the designers and decision-makers to move as one into knowing what content to write and how the website is designed around that content in order for the readers to flow through the website.
The prose on the website turns out to be cleaner, and much clearer. One goal, one message. Pages meant to inform and pages meant to capture visitors are segregated appropriately, visitors get captured and get informed. The "look better" part is just a byproduct of that kind of unity found in a start-up, it isn't even a critical goal.
As an example, navigate through a bunch of the pages of Square's main website: https://squareup.com
So if marketing is paying the bills for the website, they are going to get final say on what the website looks like. After all, they design the commercials, they write the radio ads and come up with the campaigns, why shouldn't they be designing the website to seamlessly integrate the current add campaign into the website?
1. Organizational intelligence and outlook on the web.
2. Who owns the site(s) IT vs Marketing and whom is a stakeholder.
3. Budgets.
4. Who builds it...outside thinking can produce great things but are often hindered significantly by 1-3.
1 & 2 are the biggest factors.
Established, monolithic corporations do not.