Personally, I can save up to 80% of my income. I believe normal person could save about 50% of their income, but none of countries average even 20%. Why?
If one takes a slightly more expansive view of the word "saving" to include most forms of durable asset accumulation, I think the reason some do and most don't is a matter of a few factors, I will include the three that seem obvious to me:
Most schools in the US where I live do not offer personal finance courses, and even when they do, there is no opportunity for a student to practice good financial habits in that classroom setting. I think a simple assignment that required students to track every penny that they spend over the period of a few months would help them open their eyes to how much money is spent on trivial things that they don't need. Perhaps this would be more effective in a university setting where the students are usually away from home and therefore more responsible for the spending that occurs on their own behalf.
Beyond simple education about personal finances, most people have no clue how the various financial markets work. If they understood, they would not allow inflation to eat away at their savings, but that's a separate topic from why people do not save.
Since much of the education above isn't happening, children get their primary financial education from their parents. This means that those who are wealthy teach their children how to be wealthy, and those who are poor pass on their habits to children who often also end up poor. Erroneous ideas about consumption vs. investment and its economic effects also causes some bad policy encouraging people to live beyond their means and use credit unwisely, but if you live in a country where the average person expects to eat out regularly and trade in their automobiles as soon as they experienced their highest rate of depreciation, it can be hard to recognize bad financial behavior for what it is. Collective savings rates reflect a lot of individuals who are emulating each other's bad behavior.
Even when someone is educated about finances, they may not establish good habits of budgeting regularly, tracking spending, and setting financial goals. For me, it helps to be married to someone who has similar financial goals, because we budget monthly and any major purchases (over $100 or so) must be agreed upon at the beginning of the month (with obvious exceptions for emergencies). This eliminates any impulsive spending, which is probably 90% of the battle for me. Some people do not need to account to someone else in order to spend wisely, but everyone should find a system that works for them and helps them to maintain some financial discipline.
A person who always saves and appropriately invests 20% of their income can expect to have a secure retirement. If you start early enough, you don't need anything close to 20%. Now, there are many good reasons to save for things other than just retirement, of course.
You say that you can save 80% of your income, and you expect most people could save at least 50% without problems. That's just unrealistic for most people. Taxes, rent (or mortgage payments), utilities, food, and other such mandatory expenses take far more than 50% of your income. Most people simply don't have the ability to save (or invest) 50% of their income. Or even 25% of their income.
This question is likely to be voted closed as opinion-based. That said -
In general people have become accustomed to instant gratification. They also have the media showing them luxury and are enticed every day to buy things they don't need.
In the US, the savings rate is awfully low, but it's not just the lower 50%, it's 75% of people who aren't saving what they should.
see http://web.stanford.edu/group/scspi/_media/working_papers/pfeffer-danziger-schoeni_wealth-levels.pdf for an interesting article on the topic of accumulated wealth.