It's not in any way/shape/form more practical to carry more things with me when the alternative is that someone hands me a disposable version of the thing at time of purchase.
Not everyone carries a bag with them -- and even if every single person on earth had a bag it'd STILL be more inconvenient for those people to waste cargo space in the bag with a straw when the alternative of being given a disposable one at time of purchase is available.
I feel like there is a misunderstanding of the word 'impractical' here.
Carrying your own straws is a lot of things -- wise, prepared, ethically-conscious, whatever -- it won't ever be practical ( practical : of or concerned with the actual doing or use of something rather than with theory and ideas. ) until the much more convenient option of being given a disposable straw at purchase time and trashing it at the end of use is no longer available to choose.
- You'd have to know you are getting bubble tea before you leave your apartment (or always carry a metal straw)
- It's deeper than most pockets, and can poke and potentially hurt you when bending over, so you pretty much need a bag (or use a shorter straw that will get lost in your drink).
- You need to somehow clean and dry it after use, and put it back in your bag
