There would always be underdeveloped countries, and there are various reasons for that, a few which are: corruption, poverty, political instability, low GDP, reliance on international support/aid, etc. Becoming developed is not as easy as flipping a switch, the country has to start doing things differently in order to achieve a high standard of living and TBH, it is impossible for many developing nations.
That said, there are fast-developing countries. Nations that fall under this category are making progress in one or more sectors and are generating significant revenue and making infrastructural progress, they are far from being developed, but at least they are making progress. Finally, countries hardly come out of the situation they have been in for a long time, for example, if corrupt leaders bave captured power, it will most likely never leave the hands of corrupt people and on and on it goes, except a revolution happens to break that order.