Information Disclosure is a kind of vulnerability that different people have different views on. Usually, it boils down to two competing philosophies:
It's a vulnerability, because it gives an attacker an unnecessary advantage, however small, that they would not have, would I not have disclosed this information.
It's not a vulnerability, because knowledge of the installed plugins and version in itself is not a problem, as long as these plugins themselves are not vulnerable.
Both of these views are valid, but when first confronted with them, most people tend to be on the "better safe than sorry"-side, and want to prevent information disclosure at all cost. However, sometimes it may be beneficial to disclose information, such as when scanning for vulnerabilities. This may seem paradoxical at first, but if you run a WordPress instance with a number of different plugins, having an easy way to access the version for every plugin and checking that against a list of known vulnerable plugins can be helpful.
Could all of these have been concealed?
Yes, absolutely. WordPress could have been designed in a radically different way, which would have made plugin detection a lot harder. But then again, WordPress was made by developers who primarily focussed on making a system that is easy to work with and easily extensible. Avoiding information disclosure at all cost was likely not very high up on their priority list. If it was developed by developers with very little exposure to security topics - which is absolutely nothing abnormal back in 2003 - then they might just not have thought about it.
So why not change it now? Because changing the architecture of a system that is as widely deployed as WordPress is a very, very monumental task, and such a task requires a damn good reason to undertake. Preventing people from knowing you use ACME Corp's Best Plugin Ever in version 13.3.7 is really not such a big vulnerability - even for people who consider it one.
Did the WordPress developers expose all this data on purpose?
Probably not, but I don't speak for them.
Can this still be "fixed"?
Yes, in theory. But in practice, the impact is so low and it would break so much, that nobody bothers.