Maybe a dumb question but...

Douglas fir is usually the wood of choice, probably because of it's natural structural properties, and relatively cheap and obtainable nature.   Redwood is probably not as available in other parts of the country as DF, so I am sure that at least plays into the decision for most.   If you looked at structural tables, you might be able to determine if Redwood was as strong and as flexible as standard DF?   the other possible issue that I can see is bonding,   Redwood may have different bonding characteristics that would be less than favorable in an encapsulated stringer application.    Lots of stringer jobs have been done, so a search would bring up the best ones to review.   You can get the bottom of the stringer line matched to the hull by scribing(described a few times in the stringer threads) but for the tops, make measurements first, cut second.   Take LOTS of pictures and measurements, no telling what you'll need to go back and look at after a couple of months of forgetting Any reason that you want to blaze a new trail with Redwood?   LVL is a good option, as is COOSA, and I believe yellow pine has been used too.   Maybe you're just asking the question, but probably not worth blazing a new trail on a subject that has been done successfully with other types of wood previously... my .02

If it's way more expensive why not go with Coosa board and your kids, kids, grand kids wouldn't even have to re do the stringers again?

I think the Coosa is laminated with layers & the splice joints are staggered. Search for JoeinNY's stringer thread to read more about this.

I believe everyone who has went by way of coosa has done the joint lamination process because the special order sizing of parts goes through the roof. Regardless when laminated stronger and more durable than wood.