The biggest problem might be that many players only play the Character Sheet. They are stuck in the mechanical rules. The actions they can take in the game are limited to what is on the character sheet. A spellcaster gets all kinds of cool spells to cast that can do endless things all the time during the game. A barbarian they can rage...and do some other combat effects. So, this type of player just sits there, looking at their blank character sheet, that has no actions they can take outside of combat. The spellcaster can do spectacular things nearly all the time.I am trying to think of general reasons that a player might feel their character is useless, or even just 'not awesome'.
2nd, many players lack the skill to improvise beyond the character sheet. They can only think of the game in "official game rule action moves". Ask the player, "How does your character put out a camp fire" and they would answer "my character has no fire putting out skill". And even when they are told "your character can just toss dirt on the fire until it goes out" they won't get it and will be confused.
3rd is the over half of the above. Some players can think outside the rules.......but it utterly does not matter if the DM is not on the same page. A LOT of DMs outright refuse to allow any 'character actions' to have ANY effect on the game. All actions in the game must be Official Game Actions. A wizard casts the spell Grease and the goblins slip and fall. The barbarian spills a barrel of mundane grease and the DM just shrugs "you wasted your action as that has not game effect".
4th Many DMs, including most Casual and Improv DMs, don't put much effort into the setting background and set and scenery. So PC will be in "the woods" or "a warehouse". This does not give players anything to work with as the characters are just in a vague space. Put a barrel of nails in a room and some clever players will think to use it, but just say "whatever the warehouse room has normal warehouse stuff in it" does not give the PC any ideas.
5th the final bit is many DMs don't like the idea of characters doing actions 'for free'. That is to say they only want 'important' game actions to happen when a character expends a resource. Like a spell. The character has to 'use up' the spell, so the 'reward' is an effect. But a character using a skill does not expend the skill, so it has no effect.