If phones were portable between networks, then multi-network compatible phones are actually worth selling and will show up in stores
That's already the case in the UK. Note the 'unlocked' option in the Network dropdown and the premium it brings to the price.
An unlocked GSM phone can be used with a local SIM card (so no roaming charges) anywhere in Europe, and, in the UK at least, you can buy a pay-as-you-go SIM card for less than a dollar.
Or a month's unlimited data for $25. And interestingly (for this topic) a 3UK SIM can be used in a handful of countries without roaming charges - including the USA (but data's limited to 25 gigabytes per month and you're not allowed to tether.)