Because its easier, cheaper, and less of an inconvenience for doctors to just chuck pills at someone rather than invest time in that person and find the root cause of their problem and how to help them.
Because mental health services in this country are over-subscribed, an abomination and, frankly, unfit for purpose.
Life is more chaotic, busy and stressful, and people need something to help them cope. In the UK, minimum wage is not enough to live on, and people need to work every hour they can to make ends meet. Thus having very little time to rest.
A lot of young people think they have failed in life because they don't have a six-figure salary, the body of a model, a perfect family by 25, etc, thanks to social media.
Just a few reasons I can think of.
Been on antidepressants for 10 years.