This is nicely illustrated in Marx’s tale of ‘unhappy Mr Peel’: A Mr Peel . . . took with him from England to the Swan River district of Western Australia means of subsistence and of production to the amount of £50,000. This Mr Peel even had the foresight to bring besides 3,000 persons of the working class, men, women, and children. Once he arrived at his destination, Mr Peel was left without a servant to make his bed or fetch him water from the river. Unhappy Mr Peel, who provided for everything except the export of English relations of production to Swan River. (Capital 933)
In other words, rational individuals who can acquire land for themselves, as they could at that time in Western Australia, are very likely to choose to do this rather than selling their labour power to the capitalists. So, in general, Marx thinks that the worker sells his or her labour power only because there is no real option.