Because they're out and can't replace them internally.
Makes perfect sense, and given that sound logic bomb you just dropped, now we know the exact reason why Zelenskyyidiot is incessantly begging, groveling, and pleading the West/EU/US/UK/NATO/and anyone else for more tanks, planes, artillery assets, other military hardware, munitions, etc is because Ukraine needs to replace its own tanks, planes, artillery assets, other military hardware, munitions that have been destroyed and can’t be replaced internally.
GG