Can't say exactly why for Bangladesh, but I can tell why it's so for India.

India's paramilitary forces (multiple forces collectively termed as Central Armed Police Forces or CAPF) have been kept at equally high numbers to the armed forces to prevent coups. This has been the policy ever since first government post- independence. The CAPF come under the Ministry of Home Affairs and the recruitment is done similar to police recruitment. The higher-ups come exclusively from the elite Indian Police Service, which is a bureaucratic institute with no relation to the Army. So the command structure of the Army and the CAPF are kept separate.

This in turn means, if some day the Army decides to organize a coup, the CAPF will be called into action to respond. In the past, the CAPF had been provided gear subpar to the Army. But in the last decade, CAPF has been getting nearly as good gear as the Army and have also been assigned to roles which were hitherto assigned to the Army. Now you can see them fighting insurgency in Kashmir, which until a decade ago was the Army's job mainly. This is not to say, India's goverment views its Army with suspicion and the Indian Army has been notoriously apolitical. Yet, prevention is better than cure.

Coming to Bangladesh, this is just a guess. But Bangladesh models many of its policies based on India's. The founding father of Bangladesh, Sheikh Mujibur Rahman was assassinated along with his whole family in an attempted Army coup. His daughter alone survived as she was in India at that time. Rn, she is the country's Prime Minister. I'd guess, the Bangladeshi government doesn't want a repeat of that incident and decided to have a strong paramilitary force separate from the Army as a possible counterweight.

Now, as a contrast, neighboring Pakistan has had like a dozen coups ever since it's inception. The Army holds a huge sway over the government and politics there. That's perhaps something they got very wrong.