Because Linux on phone isn't ready. Phone makers only support Android and only project that use Android kernel are anywhere near usable. Mainstream Linux still need work. PinePhone offers a good, cheap platform to keep working on it and progress is being made. Time will tell if we will ever get there. Mobile is 10x worse than Windows was back in the day. A lot of things are tied to Google so closely it will be impossible to replace (Android Auto for example).