For a class-based system that's as broad as 2e and constantly expanding with new options to work, classes have to be clearly defined in a mechanical sense, not just thematics. By being specifically focused on their particular flavour of one-handed melee combat, Swashbucklers are able to have a strong niche that distinguishes them other classes, including Fighters who specialise in having a hand free. Not only does this answer the question of "why would I build this character as a Swashbuckler?", it also helps players building a Swashbuckler know what they'll be able to do and pick feats accordingly. If the Swashbuckler had come out with feat chains to support a ranged build, a ranged-and-melee build, and so on, it'd be less clear to a new player which feats were best to pick for their character and would have meant a lot less variety for builds doing what the swashbuckler is designed to be about.
From what we've seen in playtest of the Gunslinger, it's very clear up front that its selling point is "if your character's main deal involves using a gun (or similar use of a crossbow) this is the class for you". Archetypes will inevitably let you get at some of that from outside the class, but by covering close-up shooting, distance shooting, and melee/shooting fusion combat all within this class, it lets 2e give a more specific feel than you could get by just making it a weapon type or giving every single class gun options that can be also used as crossbow options.
The other angle is that the Gunslinger will be, as far as I know, the first class to directly support melee weapon/ranged weapon dual-wielding. Given that ranged and melee characters face different risks and opportunities in combat, balancing the two options against each other is an important part of design in a game like Pathfinder. It makes sense that options to support characters flowing casually between melee and ranged would be introduced later, after designers have had more time working with the system and seeing what happens in the stress-testing of real play.
Frustrating as it can be sometimes, when you're bringing an existing character concept into a system like 2e that has a lot of mechanical parts to put together, it's important to consider what you want the character to do and how the game mechanics let you represent those actions, rather than what class names look most fitting on the sheet or what cool options exist but don't quite come together into the character you're trying to play.