The smallest camera I can pocket weighs 5x my phone, is about 10x thicker.
GPS, same.
Mp3 player, about the same as my phone.
Computer/web browser? Well, nothing is as small as a phone.
I get all that in a single device with a phone weighing 8oz, measuring 6"x3"x3/8".
Separate devices is better if your use-cases for them have strong independence (e.g. Only use GPS in the car/on motorcycle, only use a camera when doing dedicated photo shoots, etc). If anything I’d say multiple devices is less convenient even then, it’s just that those devices work better for those use-cases, making the tradeoff of less convenient worthwhile. I’d much rather use a dedicated camera sometimes (and do), when I’m taking lots of pics and want to go faster.
But for most people, these activities are strongly related, and occur throughout their day. It would be far less convenient to carry multiple devices and have to pull them out and handle for these activities.