There are several main reasons why oil still doesn't have adequate alternative and continues to remain such a dominant source of energy. Through this article I will point out some of these reasons that will explain to you why the world still so heavily relies on oil.
1. Powerful oil lobbies. Long tradition of using oil has resulted in powerful oil lobbies that have big influence on politics, and they use this power over politics to make sure that major political decisions go their way, and with it they slow down the development of renewable energy technologies.
2. The young age of renewable energy. Renewable energy still needs to mature on technological level because current renewable energy technologies are still far from being competitive with oil and other fossil fuels in terms of costs and efficiency.
3. Renewable energy costs. Renewable energy is still more expensive option compared to oil and fossil fuels, and many people are still not ready to pay more for clean energy. Going green may look like a widely popular idea but for many people this idea is only popular if they are not required to pay more to go with it.
4. Intermittency issue of wind and solar energy. Two most popular renewable energy sources, wind and solar energy, still suffer from intermittency issue, and they require cheap and efficient energy storage methods to ensure reliability in times when sun doesn't shine and wind doesn't blow.
5. Nuclear energy still looks unsafe for many people. Though some energy experts see nuclear energy as one of the best alternatives to oil and other fossil fuels many people still look at nuclear energy as the unsafe energy source that could result in yet again nuclear disaster like this was the case with Chernobyl and Three Miles Island. Though new nuclear power plants operate under conditions that should guarantee maximum safety and zero chance for nuclear reactor explosion there is still matter of what to do with nuclear radioactive waste that remains radioactive for around 5000 years.
6. Geothermal energy has high installation costs, and is really economically viable only near the tectonic plates. Geothermal energy doesn't suffer from intermittency issue like this is the case with solar and wind energy but doesn't belong to energy sources that could be used on global scale. Well, at least not with the current technologies and current costs.
7. Increased use of biofuels could increase food prices, and even cause food shortage. First generation biofuels (made from food crops like corn) could significantly increase food prices and lead to even bigger hunger in the world.
As you can see I have pointed out some reasons why certain energy sources are still far from being ready to replace oil in near future. This does not mean that oil is totally irreplaceable, what this means is that each country needs diversified renewable energy portfolio thus relying on two or more renewable energy sources to make the difference.
But of course the science will be the one that will decide the final outcome, and in the end the most important thing would be whether the science will be able to make renewable energy technologies competitive in terms of efficiency and costs with oil and other fossil fuels.