Why many companies apply pre-employment assessments to evaluate candidates based on Psychometric assessments rather than selective assessments?

Probably because it is the logical way to decide who is the best choice for the job.

In or co psychometric tests are conducted to test the integrity and mental balance of the person and ability of the person to handle stressful situations without loosing ones cool. 

It consists of a number of questions which seem slimier but take out the inner thoughts of the candidate. It is said u cannot fake a psychometric test. 

Relying on psychological assessment in hiring process could help in evaluating and guidance;For example:

1. The capability of employee in handling stressful situation.

2. The mentality status of the employer/candidate.

3. The ways of thinking.

4. The right action and reaction in certain situations.

5. Other psychological metrics

1) For ensuring the capacity to carry on with the job with his/her skills and knowledge 

2) To identify the balance between his/her mental condition and handling the situation.

3) To be sure of his / her communicative and behavioral approach.