The goal of a URL is not to mirror your site structure. There are four goals of good URLs:
- Uniquely identify the content of the page
- Be descriptive
- Be memorable
- Be permanent
Sites include a numeric ID in the URL because it works well with databases. The numeric id is a database key for the content. This site works that way. When that is the case, the only portion of the URL that generally matters much to the software running the site is that ID.
Any words that are added to make a URL descriptive are called the "URL slug". The webserver need to handle cases in which the slug changes. It either needs to redirect, or put canonical tags in the page for alternate URL slugs.
To be memorable, a URL needs to be very short with no numbers in it. The path should have only one, two, or three words. Most sites don't try go for memorable URLs these days because most users rely on search engines rather than typing in URLs.
A flat URL structure works very well because most sites don't need categories, especially nested categories, in the URL to uniquely identify or describe the page content. Categories also introduce additional permanence headaches. When the name of a category changes, many URLs would have to redirect when the category is in URLs.
It also seems to me that the guides that you are reading are out of date. A tree structure is no longer state of the art for web information architecture. Users prefer faceted navigation. That is, different users will have different criteria for finding the same products. To use your ticketing example, users might want to look for tickets by:
- Category (sports, music, theater, etc)
- Artist, team, or performer
- Popularity (big events or cozy events)
Websites may have multiple ways to navigate to the same product with different ways of starting out. Many sites implement multifaceted filters, usually in the left hand navigation. When using multifaceted navigation, the URL structure can't match the path the user took to get to the product because there are multiple paths to the same end.
As far as SEO goes, search engines put very little weight on the words in the URLs right now. Years ago, it was an important ranking factor, but these days it seems to make little difference. As long as URLs are unique and consistent, the presence of keywords seems to make little difference for rankings. Many sites ranking very well without any keywords in the URL. Putting keywords in the URL might give you a very slight SEO advantage. Putting those keywords near the front of the URL also might be a very slight SEO advantage.
If you do use a numeric id, it should go near the front of the URL. In my opinion this is far more important compared to putting keywords first. URLs often get truncated by email or when displayed to users. When the ID is at the front, it is easy for your web server to figure out where to redirect a truncated URL. Picking up a few extra backlinks that would otherwise end up on error pages because the link is truncated far outweigh the benefits of the keywords.
Another reason that URLs within an industry tend to be similar is that some site does it one way, becomes the industry leader, and everybody copies them. To some extent this works OK, because the leader isn't usually doing anything really bad from an SEO standpoint. It does mean that within a particular sector, many of the sites will share odd quirks that may not actually be helping them at all. My guess is that is the case for Stubhub and Ticketmaster. It really looks like one of the sites emulated the URL structure of the other.
Of your examples, the best URLs for SEO in my opinion are the SeatGeek URLs. They have the descriptive keywords. They don't have extra fluff. I could probably type them in. The even try to handle misspellings. If I change the spelling to "diago" in the URL they try to take me to an appropriate page (although their logic needs some work because it takes me to a page for 49ers tickets).
None of the example URLs you gave are "bad" for SEO. I'd say that each of them is good enough. Google is likely to rank those sites pretty equivalently as far as the URL itself goes. Other ranking factors are going to have much more impact than the URL.