Not sure exactly why you are asking this or how it will help, but I'll give it a shot. Very likely the production of more than 1000 good, well-ranked results for any given query would require more resources than google really wants to put out - there aren't many 'people' who are going to go beyond the 1000th result. There are, though, many bots that would be willing enough to do this if you configured them this way - and this might be a second reason. Probably most of the queries beyond 1000 were coming from bots.
This is of course only speculation - the real answer can probably only be provided by someone who's been informed 'by the horse's mouth'.
probably takes extra processing power. no sense wasting extra processing power and requiring extra servers just to satisfy someone who probably is violating the terms of service of the site by sending in a bot.