Land speculation was considered a lucrative investment for the gentry during the latter decades of the 18th century and in the early 19th century. This was especially true in the upper South, where tobacco prices were consistently low, other cash crops were difficult to consistently produce, and the monetary demands of their lifestyle meant most were heavily indebted to British (mostly Scottish) creditors (see Woody Holton’s Forced Founders for how this influenced elite participation in the Revolution). Land speculation was so vital that the Proclamation of 1763, limiting English settlement to east of the Appalachians, was a major driver of American resistance to the Crown across social class. For the gentry, it threatened them with bankruptcy, since many (including Washington) had already heavily invested in the Ohio Valley.
Properly speculating and investing in land meant being able to establish strong legal claims backed up by verified maps and plots determined by accurate surveying. For men like Jefferson and Washington, who grew up near Virginia’s western edge of substantial European settlement, it was a given that they would want to invest in land and protect that investment with proper surveying.
For Lincoln, living along the western edge of settlement in the 19th century, concerns about accurate land claims meant surveying remained a popular and necessary occupation. It was no longer a gentleman’s prerogative by the 1830s, but was highly valuable. In Lincoln’s particular case, his family left Kentucky because that state based land claims on landmarks rather than traditional surveys. This meant that claims were often contested and lost as the landscape changed and those with power and connections could abuse the court system to their advantage. Lincoln’s father lost his first claim and eventually moved the family to Indiana to find a more stable government.
Something to keep in mind and to add to what u/Toroceratops provided is that it was much more the gentry profession thing than it was Proclomation of '63 or land speculation. While those absolutely put Washington's holdings in jeopardy, he had started surveying as a teen at Mt Vernon as a continuation of his studies in mathematics. His neighbor happened to be William Fairfax, and in the 1740s he tagged along with him for a major surveying expedition made at the request of William's first cousin, Thomas Fairfax, 6th Baron Cameron, better known simply as Lord Fairfax, the man who at one point owned most of Virginia including massive holdings in the Blue Ridge Mountains.
Fun Fact! Parking in a small lot at Bootens Gap off of Skyline Drive in Shenandoah National Park, you can take a short hike down Conway Fire Road and burried in the underbrush just off trail you can find a small stone with a plaque marking this point of the Fairfax Line.
With a big massive boundary of his holdings set, they needed more surveys and a 16 year old Washington joined the effort, the start of his professional career as a surveyor. His last survey happened in 1799, again at Mt Vernon, and only 6 weeks before his death. He surveyed tens of thousands of acres in his time as a surveyor, and served in that official role in Culpeper County. He's also said to have thrown a stone over Natural Bridge, a 215 foot high natural rock formation bridge in Virginia, from the creek below (and you can still see a "GW" carved in the rock, allegedly from the man himself when he surveyed that land).
Segway.... Jefferson purchased Natural Bridge later on and held it for some time, desiring to but never actually making a home on the site. His survey story is similar, and also starts in the late 1750s. His father, Peter, was never educated but was self taught. He paired up with Joseph Fry, who was an official surveyor, and worked under him under a project in 1746. Pleased with his skills, Fry and his new partner set out to define the border between VA and NC. Accomplished, they next set out to map the colony of Virginia and this project, finished in 1751 when TJ was only eight years old and running barefoot at Shadwell, was the first map to include the Allegheny Mountains (first published in 1753 as the [Fry-Jefferson Map] (https://www.loc.gov/resource/g3880.ct000370/)). Fry passed the next year ('54) and as one of his final acts he bequeathed his surveyors tools to his partner, Peter Jefferson. Jefferson was done surveying himself as well, having returned to Shadwell and becoming a magistrate for Albemarle County. He died only three years later, in 1757, and his tools plus those of Fry (likely) all went to his son, Thomas (we know at least some did but believe they all did). He said of his father;
My father’s education had been quite neglected, but being of a strong mind, sound judgment and eager after information, he read much and improved himself insomuch that he was chosen with Joshua Fry professor of Mathm.[Mathematics] in W.[William] & M.[Mary] college to continue the boundary line between Virginia & N. Carolina ... and was afterwards employed with the same mr Fry to make the 1st Map of Virginia which had ever been made, that of Capt Smith being merely a conjectural sketch. they possessed excellent materials for so much of the country as is below the blue ridge; little being then known beyond that ridge.
Another tale, this one about the border survey of VA/NC, was passed down to and then from TJ's grandkids;
Colonel Jefferson and his companions had often to defend themselves against the attacks of wild beasts during the day, and at night found but a broken rest, sleeping – as they were obliged to do for safety – in trees. ... Jefferson's courage did not once flag, but living upon raw flesh, or whatever could be found to sustain life, he pressed on and persevered until his task was accomplished.
Jefferson the son held his father in high regard, had his tools, and possessed a high quality education for his time and area. And, unlike Washington, he spent his whole life on the frontier of English settlements, at the absolute edge of the empire. His choice of profession was almost preset, but not satiated with that he chose to pursue a legal education while in Williamsburg, later also following in his footsteps of becoming a judge in Albemarle (Washington also served as a county magistrate, btw).
So it was primarily a result, at least in the case of these two, as being a gentry profession that was almost a given to continue ones studies of mathematics. The fact that these specific two went on to be so famous is happenstance, yet the odds of a 18th century surveyor being an influential person was pretty good. Neither would ultimately define themselves as surveyors, being a stepping stone early in their careers.