Because California's already done it to the point already where, despite the population being 38,28% Republican and 58,38% Democrat (with the remaining 3,34% being independent or third party), Republicans only have 9 seats as opposed to the 43 the Democrats have. This means 38% of people have about 17% of the representation, and 58% of people have about 83% of it.
If California didn't district and instead divided its 52 seats equally, the Republicans would have 20 of their seats while Democrats would have 30, with the remaining 2 seats being granted to whoever the independents voted for.
California has already stolen pretty much half of the seats Republicans would've gotten and all the seats of potential third parties if it weren't for gerrymandering, and this can be mathematically proven by the calculations I'll put below.
Non-district based distribution of seats, using only the total amount of votes of the 2024 election.
Republicans: 52/100 X 38,28 = 19.9056 (rounded up to 20)
Democrats: 52/100 X 58.28 = 30.3576 (rounded down to 30)
Third party: 52/100 X 3.34 = 1.7368 (rounded up to 2)
Considering how Dems have 43 of the seats and Reps have 9, something here doesn't seem to add up.
Now, let's do the same for Texas, just to see the difference.
Republicans: 38/100 X 56.03 = 21.2914 (rounded down to 21)
Democrats: 38/100 X 42.37 = 16.1006 (rounded down to 16)
Third Party: 38/100 X 1.60 = 0.448 (rounded down to 0)
Now, using this system, there'd still be 1 more seat left, so let's give that one to the Third Party for being much closer than either Republicans or Democrats to getting rounded up.
Now, seeing as there's in reality 25 Republican seats and 13 Democrat seats (counting the vacant one as that one was to a late Democrat), there's still a discrepancy that I don't like, but said discrepancy is much smaller than the one in California. If both states removed districts altogether and used the calculations I just made, Republicans would still gain a total of 7 seats while Democrats would still lose 10, with the remaining 3 seats going to the Third Party.
Honestly, I don't like gerrymandering, no matter what party does it. However, from what I've seen, Dems are way more guilty of it, even if Reps aren't blameless either. That said, the only way to stop it is if both parties agree to do so. Or if you just eliminate districting altogether, and simply divide the seats based on vote percentage like I just did.