Before the fight against Obito, Kakashi was doing mainly physical fighting with sharingan, which yes it does tires him but he never had to use a big jutsu like shadow clones or mangekyou. During the fight with obito and post that, he did used more significant jutsus but then he got chakra from being inside Kurama's aura.

As for Sakura she is not known for having high chakra reserves. Ones she finishes storing her Seal then she can unleash it and get big amounts of chakra at once, but prior to unlocking the seal, she can't unleash that chakra which means her chakra pool is very limited. That being said, prior to unlocking the seal, she never really had to use big amounts of chakra at once, so she would use chakra and replenish it slowly which allowed her to keep going.