S1 of the Theft Act 1968 defines theft as "dishonestly appropriaing property belonging to another with the intention of permanently depriving the other of it". If you went into a store and took a copy of Windows XP without paying, that would be theft because you would be depriving the store owner of a tangiable asset. However, by taking a copy of software without paying for it you are in no way depriving the owner of the original property and therefore cannot be accused of theft.
Of course this is all UK-based law, but I'm fairly sure the situation is similar in the US. Theft is not sufficient to cover software piracy.