Well you are correct a lot of tasks require strength AND endurance, especially sports and competitions of sorts, but people just get confused, once you are strong enough to complete a task once you now have enough strength to do it, if you want to keep repeating the task no further strength is needed IE you do not need to get stronger you need to have muscle endurance. Hence why we add weights in weight lifting to increase strength.
Doing weight training and most other activities will build up both strength and endurance, so doing sets of 10 and adding weight when it gets easy will still get you stronger, but getting stronger will only have limited carry over to endurance. A person that can bench press 400lb x1 can not necessarily do 100 push ups or vice versa, there is very little carry over.
I often hear people say I am not strong enough to to this or that, and most of the time they talk about endurance tasks such as doing lots of push ups, chins, sit ups etc. Doing 500 push ups does not really demonstrate any more strength than doing 100, it shows the muscles ability to perform a given task repeatedly.
If you get a 80kg martial artist that does not train weights and a 80kg strength athlete (Oni for example who has strong lifts) you will find the martial artist will be able to to a crap load of body weight push ups, dips, squats etc but his strength will be no where near that of Oni. Both train for different things, and both will on average perform better at a given task than someone who sits and drinks beer all day long, both both will excel in different things.