Dlmr, the best way for me to respond is to explain how prices are relative to incomes (as well as them being dependent on the state of technology in a country).
There is an alternative answer in that it would be practically difficult to have a universal monetary policy. The reason why different currencies exist is because it is politically difficult to co-ordinate monetary policy across several countries, let alone the entire world. Because different countries have separate monetary systems, with different currencies, there end up being differences in how valuable each currency is. The value of a currency derives from how many goods and services that country trades with other countries (as
Herr K. says, "poor countries ... cannot produce enough valuable goods and services").
In everyday life, people are generally not affected by prices in other countries. If a Coke costs double in an overseas country, so what? You don't go there often, so it shouldn't concern you. However, as with your holiday, people sometimes venture to a different country, and the exchange rate does make them think and wonder about why things cost less or more. The key here is that you only consider things to cost less or more because of your salary. The salary you earn is in a different currency, in a different country. Because it's in a different currency, the exchange rate affects what you can do with that money in a different country. But because you work in a different country, your employer will pay you what you need in order for you to live in your home country! (Employers don't generally think about it this way, but this underlies the level at which wages are set. Your employer will more likely observe the prevailing level of the wage for your type of job in the labour market and not deviate too far from that level.)
So, I struggle (because of what I've learnt in economics) to understand what a more "expensive" life is. A life being more "expensive" sounds as if the person is poor and cannot afford to have as much. It depends on what job they have. If they have a job that actually pays what you would think is a "lot" of money, but that enables them to pay for "expensive" things, that would in my opinion be normal for somebody living in that country. It just means that prices are higher, but it also means that wages are higher. Perhaps what is confusing you is that their money can buy more in a poorer country (in that case I refer you to my argument above that employers simply pay wages that are needed in order for their employees to live in their own country).
I believe that people living in a poor country generally can afford plane tickets, houses, food, etc. It's just that the strength of the currency they use is low when traded with other currencies.
Now, after talking about how prices are relative to incomes, I want to go back to the important proviso in the brackets in my first sentence. Technology can certainly account for differences in living standards across countries. For example, cell phones, laptops, cars, production processes, infrastructure, human expertise and business management are technologies that can be more difficult to implement in a undeveloped country, or they can be technologies that are still in elementary forms in those countries. This is a game changer, because technology has a big impact on livelihoods, and the ability of people to access the same quality of life as those in richer countries. This is maybe why you think lives can be more "expensive" - because the same level of technology take a comparatively larger proportion of those people's incomes.