Steam boats didn't come until long after colonies had begun being set up by Europeans. And China DID colonize. They clonizes China. They had loads of dynasties and foreign invaders pushing down on independent cultures, and they had a syste of tributaries, where certain states had to go to the Chinese capital to ask for the okay on their new rulers when the previous died. China also wasn't quite as well placed for the colonialism of the sort you might be thinking. The Baltic, the English Channel, the Mediterranean.... Lots of places to bring use ships, meaning less distance on food or by barge to get goods to the coast for international trade. The Chinese (Of course this is dozens of governments over thousands of years, so their motives and rulers changed, but in general-) did not see others having things they wanted, outside of food and luxury goods. There is a reason the Europeans kept going to East Asia. They had a lot of nice stuff. The Europeans didn't have much they could bring over great distances besides precious metals and stones, which was why the Dutch made a lot of their money trading Asian goods between semi-isolated or hostile countries, then using some of the profits to send home spices, silks, porcelain, etc.