Oh theres so many reasons. The highlights?

Theres a strong probability that you are time poor, this kicks your ass in two ways. You get a lot less incidental movement. You drive more and walk less, order things to be delivered instead of going and getting them. Secondly sacrificing sleep which leads to...

Testosterone dropping, not only does your testosterone start to fall in your 30s, low sleep has a terrible impact on T production. Which means a whole bunch of lovely side effects including lower lean muscle mass, depression and fatigue. Oh also being overweight also fucks your T levels...

Lets throw in the fact that most of us have indoor jobs that are very sedentary, which ruins your vitamin D levels. Which leads to fatigue, depression, poor sleep (theres that sleep again) and a bunch of other shit.

The thing is its actually not that hard to stay in shape. Its about a sustained conscious effort and finding things you like that work FOR YOU. Theres no secret diet or workout plan. Calculate your Macros and eat accordingly, find an exercise you LIKE doing and do it often. Dont have a vision board or a body goal, learn to love the process and its the easiest thing you can ever do.