From a simple Google search:
Thomas had eight siblings, and was the youngest child.
While the rest of the family's sons pursued military careers,[12] the family intended for Thomas to follow his uncle into the abbacy;[13] this would have been a normal career path for a younger son of southern Italian nobility.[14]
And biography.com asserts,
Before St. Thomas Aquinas was born, a holy hermit shared a prediction with his mother, foretelling that her son would enter the Order of Friars Preachers, become a great learner and achieve unequaled sanctity.
You're trying to make this about him as an individual, when in fact it seems like a combination of family needs and inheritance customs/law that even a hermit could predict in advance.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.