Consistent effort over time gets results. This is true in all life, of course, but is especially true in physical training. Most people do not apply consistent effort. Someone might do 10 pushups, then struggle out 5 more, their body peeling up from the ground like a band-aid, their face fierce as they grunt and strain, then 4, 3, 2 and 1, and collapse; the next day they can't comb their hair, and they do not do pushups again. It would have been better for them to just do 10 pushups every day. This would be consistent.
The body is not a machine. It repairs itself, it adapts. If today you do 10 pushups and it's hard, your body says, "that was hard, I'd better build the capacity to do 11 pushups, then next time it won't be so bad." If tomorrow you do no pushups, your body will say, "I built this extra muscle, and it's not being used – back to being able to do 10 pushups," and that extra muscle will disappear. If instead you do 11 pushups, your body will have a reason to build again. And then 12, and so on.
This is fancily-called the SAID principle, Specific Adaptation to Imposed Demand. In plain English, If in every session you do more than you did before, you will get stronger, fitter, and your physique will change. For fitness, you must go faster or against increased resistance or do it for longer. For strength, you must lift more weight, or the same weight more times. For mobility, do an exercise or stretch over a greater range of motion, if you were doing quarter-squats do half squats. Do more.
Most people in the gym do not make a consistent effort over time, and do not do more in every session. Most don't have a routine, they just wander around doing a few of their favourite exercises with a few more random ones thrown in, something they saw someone else do; their efforts are inconsistent. How then can they know they're doing more? If I squat 60kg for 5 reps today, and leg press 80kg for 8 reps tomorrow, was that more? But if I squat today and squat tomorrow, I will know if I'm doing more.
Of course, doing the same thing all the time is boring, right? I had a conversation with a client a while back.
"These workouts we're doing are great, but what about the workouts I do on my own, could you give me some more exercises? It's getting boring."
"Try putting another 5kg on the bar."
"Oh okay." Two days later, "That was great!"
If you're bored, you're not working hard enough. Those who do stick to the same exercises tend not to do more. Go to any gym and look at people's programme cards, you will see the same weight, the same sets and reps, for weeks or months on end. If the person is happy with their fitness, strength, mobility and physique, that's fine – but if they want an improvement, they must do more. The problem is that doing more is fucking hard.
Consistent effort over time gets results, and to improve you must do more than you did before. This is simple, not easy.