There are so many reasons behind the joblessness of mechanical engineers (include other engineers too) in India:

1. Lack of skills : Today, every 3 candidates among 5 are engineers. But still, if we search for a skilled one, it's very difficult to find a skilled engineer. Reason it, they have degree only, but in terms of knowledge, most of them are useless. Getting a degree through distance education mode or by clearing backs in colleges doesn't make anyone an engineer. I'm social media, there are a lot of pages who make memes about engineers. I've seen in matrimonial ads too, that a few people mentioned that they aren't looking for any “engineer” for their daughters (funny).
2. Outdated syllabus taught by colleges: In colleges, the syllabus being taught is far behind. They aren't up to date with the latest trends and industry demands. So even a bright student, after leaving college, finds it very difficult to qualify an interview and to get a job.
3. Reservation system and politics: We have reservation system and politics to blame for any reason, at least. There are several government jobs too in the field of engineering but it's sad that the ones who get selected for the job or mostly dumb and have no idea what to do in the specific field. The reason is, yes, reservation, and the other reason is, exam patterns for govt jobs. In the entrance exam for engineer post, they ask stupid questions in the name of G.K. and current affairs which are total useless in real life. A serious engineering student doesn't care about GK etc and focuses on his main subject so when he appears for the government exams, he fails to secure cut off marks because for general students, cut off is very high and a reserved student gets benefits because the cut off for them is very low and even if he leaves all the GK and other stupid questions unattended, he qualifies.
4. Very less salary: In private sectors in India, the salary for a fresher engineer is very less. This is one of the main reasons engineers don't survive and keep changing the jobs and out of frustration, they quit and go for business. The ones who belong to rich families and are good in their field leave India and get a job in any company outside India with a good salary.
5. Population: Today, there is a tough competition in the field of engineering and we have a lot of engineering graduates who are jobless. The country has limited jobs available and it's not possible to provide jobs to every single graduate.

There are still other minor things which are the reason of joblessness of engineers. But there are no less opportunities for skilled and talented engineers. If you have the skills, interview in different multinational companies and prove yourself better than others.

Hope you are doing well

As you know some mechanical engineers are doing very  well in their respective fields . but if you observed that the demand of mechanical engineer is less as compared to college seats. If you take example of government sector if you see vacancy you will find that only 10% seats are for mechanical engineering this is because of less mechanical engineering work .so this can be a reason for your answer