While to the casual observer it seems like most of the doors around them are plywood doors, this is just not the case. The preferred choice for making wooden doors these days is Block board 'Flush Doors'. These Flush doors have solid rectangular blocks of softwood on the inside arranged end to end, and covered on both side with a very thin layer of ply about 2.5 mm thick.
Theoretically speaking, a door can be constructed entirely out of plywood, but this is rarely done in practice. The reason for this is that lengthy pieces of plywood are very likely to bend at the center. On the other hand blockboard flush doors do not usually warp in such a manner.
Secondly plywood is usually made from hardwood which makes the board is quite heavy. Hence plywood doors will be much heavier than flush doors (that have softwood blocks inside). Heavier doors can be much more difficult to transport and install, and to operate in day-to-day use.
The third point of comparison is material cost. Plywood is costlier than blockboards, and hence the cost of Plywood doors made completely out of plywood will be much more than the cheaper flush doors.
A word about Flush doors
Flush doors are of two types. Solid core and hollow core. As the name suggests Solid core flush doors have solid blocks of wood inside, and the hollow core doors are hollow on the inside with only a supporting honeycomb structure. Solid core doors are stronger and have better sound proofWhile most doors of houses are made from solid core flush doors, the hollow core doors are considered suitable for making partitions.
Entry doors of houses are usually made from BWP grade flush doors. As per the Indian grading system, the acronym BWP stands for Boiling water proof and these are of an exterior grade waterproof quality.
Many of the major plywood companies in India also manufacture ready made flush doors, that are ready to install and don't need any further finishing. Such doors are available in a variety of attractive designs and colour shades. Good quality wood veneers can also be used as an overlay to enhance the look and feel of doors.
To summarize this article, I would say that all-plywood doors are a rarity, while flush doors are currently the in-thing.