You’re right that it is easy to use Coinbase, if Coinbase supports you.

Coinbase does not support every country in the world. For example, it only “supports” 13 countries in Africa. Of these 13 “supported” countries, none of them are able to purchase cryptocurrencies through Coinbase, instead only supporting conversion between currencies that they already have. In fact, Coinbase only allows people in 44 countries to purchase cryptocurrencies.
https://www.coinbase.com/places

Even in the countries that are supported, Coinbase only works if you have a bank account which many people in the world do not. Of the people in the world that do have a bank account, many of them do not have reasonable access to online banking or the internet due to either lack of infrastructure or lack of education to use such a system.

Even if you do have the support, infrastructure and knowledge needed to purchase cryptocurrency, you do then need to have enough money to purchase it. This is a situation which many in the world cannot afford.

I’m not trying to say the Coinbase isn’t great for the people that can use it. Cryptocurrencies are definitely not available in an easy format to too many in the world — generally, the ones that are already in a financially poor situation, where this kind of disadvantage may make it worse.