Without something like the GDPR, incentives are very different for exporting vs importing: Import functionality makes it easier for someone to switch to your service, while export functionality makes it easier for someone to switch away.
Additionally, if a service is missing import functionality, you can choose to not use it. While you'd often only realize you need export functionality while you've already used a service for years.
Maybe the case of porting data between social media services doesn't make much practical sense, but there are many other services where it does. Fitness tracking apps as an example - you'd likely want to take your history with if you switch services.
And usually these services would not be using the exact same data format for import vs export. But that's not as big an issue in practice - you'd often find third-party scripts or services that can do the conversion for you.
Additionally, if a service is missing import functionality, you can choose to not use it. While you'd often only realize you need export functionality while you've already used a service for years.
Maybe the case of porting data between social media services doesn't make much practical sense, but there are many other services where it does. Fitness tracking apps as an example - you'd likely want to take your history with if you switch services.
And usually these services would not be using the exact same data format for import vs export. But that's not as big an issue in practice - you'd often find third-party scripts or services that can do the conversion for you.