Think of it like a shoe, the question would translate to,

Why is there no universal shoe size?

The answer is simple, right? Because there are many different varieties of feet, shapes, sizes, gender etc.

With countries, it is the same thing. Different countries have very different lifestyles, affordability(Purchase power parity to be exact) and thus cannot afford the same currency because it would create a huge mess in terms of denominations required.

Your question is already in effect for some countries. The Euro is used in most European countries because these countries are similar on economic terms. All of them huddled together have comparable statistics on many scales, even lifestyle for that matter. I'm not a European, but I'd like to think that with a certain salary, the average lifestyle of a person should't deviate too much from Spain to Italy lets say. As such, these compatibility features enables them to follow a single currency.

Back to exchange rates, 1 US dollar can get you a small water bottle in the US, but in India, it will get you a full meal and more. In certain African countries, your dollar is worth a lot more, and hence the average person will have to deal in cents, even denominations that extend to lower than a cent. This means that while we're printing dollars in some countries, we'd be passing around cents in other countries. Literal whole countries would run on just cents as their day to day currency for liquid market trading.

Any currency rate depends on a lot of economic factors. Some important ones are import/exports, growth rate, poverty rate, employment rates, Foreign investment funds etc.

That's the reason developed countries have currencies that "compete" with each other. 1 U.S dollar is currently 0.81 pounds, 1.32 Australian Dollars, 1.01 Swiss Francs, 0.94 Euro and so on. You can see that all these are comparatively similar. There are a lot of exceptions obviously, the Yen is one of them (1 USD = 114 Yen), but that is happening because of other reasons than the ones I've mentioned above. The Russian ruble is also a interesting story, it fluctuates like crazy.

Now lets look at developing countries. On the top of my head, countries like China, India, Brazil come to mind. Their current exchange rate is 6.9 Yuan, 66.7 rupees, and 3.9 Real respectively, which isn't too bad. India's far off the game, but not because they haven't had growth, its because they have extreme poverty and corruption among other factors.

Underdeveloped countries, thats a whole other ballgame. 1 USD in the world's most underdeveloped countries mean a LOT more than it would in the US. The craziest I've seen is Zimbabwe introducing a 100 trillion dollar bill in the market because of the hyperinflation.

In laymen terms, countries are just too different to be following one single currency. Unless there comes a time where all of us have countries that are very similar to one another, the concept of a global currency is not imaginable.