Can you improve during the season? Yes. But remember you only have so much ability to recover. When playing a sport that lifting is simply GPP, you have to temper developing maximal strength with the ability to recover from a variety of different other skills/needs of the sport.
For someone squatting 700 lbs as a collegiate football player, does it serve them better to "use" their recovery ability to get to 750, OR, spend it on improving their speed, their agility, their technical prowess at their position? Chances are it's the later rather than the former. Remember the best player isn't necessarily the strongest player, it's the player that excels at many things (not just mediocre at a lot of things like crossfit), and brings the full package that ends up playing in the pros.
As a side note, Dr. Yessis, Dr. Issurin, Dr. Verokshansky are all quality sources for bloc periodization - which is a great model for training in-season and keeping the gains going.