The same reason people have been speculating for years what is going on the MIB plot. It is fun and probably none of the ideas we are talking about now will happen. Phase 4 would be so far out that who knows what will be popular in the Disney world of IPs. So even if it does happen, it probably won't be the IPs we are talking about. Another Frozen could pop up.
I think there will be a Phase 3, because we know 1 & 2 are happening for sure. Also, because the park will have extra land after phase 1 & 2 based on the changes they are making and things that can go bye bye. But I am not sure it will happen right after Phase 2. I am wondering if they might tackle some other parks before moving onto phase 3. So I see probably at least 5 years between completion of Star Wars and Phase 3. So phase 4 would be really out there. But I do think the infrastructure changes they are making now are with a phase 3 and 4 in the plans eventually. I think they learned their lesson to not land lock themselves like they originally did.
Keep in mind at the beginning of 2016 they will have 3 things in their resort under construction. Pandora, Toy Story, and Frozen. This is pretty different for Disney from what we have seen in the past. 3 different parks 3 different projects all happening at the same time. So I have more faith under the current management team that things are going to change. If rumors are true, by the end of 2016 Star Wars will start up too.
Also, when is the new film for Soarin occurring? The only park not being touched is MK and most likely because it just got stuff and got the hub re-do. I think that will be tackled after Star Wars which is doing something with Tomorrowland. That place needs the most help right now.