Long term travel without being wealthy - how is it possible?

I recently travelled for 6 months through Asia. During this time I met many travellers who were travelling for much longer periods, some for several years.
Most people like myself had simply worked and saved hard for a year or more then travelled through counties where it is cheap. I also met a couple of people who were working on their travels, a writer and an entrepreneur.
In South America and Asia you can travel very cheaply ($30USD or less). As long as you are willing to live frugally and save a good amount before you go you can go for a long time.
When you travel on a long trip like this you have 3 main costs.

• Living costs. Food and accommodation. Every country is different but in places like India, Cambodia, etc this can be very low.
• Site seeing. Prominent historic sites often have high fees. Especially if you are visiting multiple locations every day. Choosing to only see the most prominent and the cheapest can decrease costs.
• Internal Travel. Flights and trains from one city to another can often be expensive. These can be mitigated by staying in each place for longer.

So if someone saves say 10,000USD they could easily travel Asia for a year. Of Course that's a lot of money, but if you don't have children, a mortgage etc, and save aggressively it is very possible. The long term travellers I met came from all works of life, writers, programmers, nurses, students, factory workers.
A final note is that if you're alone your costs are higher. A couple can halve their accommodation costs as well as taxi/tuk tuk hire.