Why do countries like Pakistan, India, and Bangladesh have a parliamentary form of democracy?
These countries basically all use the Westminster parliamentary system of government which is modelled after the UK parliamentary system.
This parliamentary system is mainly used by former British Empire colonies upon gaining independence and Pakistan, India, and Bangladesh were all formerly ruled by the British. Both India and Pakistan gained independence from Britain and Bangladesh then gained independence from Pakistan.
Since they did not reform their system of government, they are still using the Westminster system of governance till date and thus they do not have the presidential form of democracy.