That's not quite right.
While nearly half of all habitable land is used to some extent as agriculture, the total amount of land in active cultivation has actually been declining since the early 2000's and we currently abandon more farmland every year than we clear forest or convert to agriculture. Between 1992 and 2020, ~ 7% of all cropland was effectively 'abandoned' and much of the production capacity in the west is actually intentionally kept from being fully planted in order to control against overproduction of cereal crops which in the past has been justified by arguing that this control is necessary to prevent market crashes when distribution networks cannot absorb the full capacity of a harvest.
Globally, there are about a billion acres, 400ish million hectares, ~ about half the size of Australia, which sits unused according to FAO satellite and other observables datasets.
About half of the abandonment not due to cultivability at the simplest level, but rather that of transportation of the crops to market or of access to workforce (labor or equipment) to the land. Smaller, more distant crops without significant mechanization simply cannot compete at the prices possible through mechanization. With a lack of access to global niche markets and the higher prices for, say, premium organics, many successive generations of farming families have simply moved to town and left the land sitting, unused.
The other ~ half of abandoned land is because of degraded production capacity through mismanagement; either through over-application of petrochemical fertilizers instead of crop rotation, or through mechanized tilling without careful erosion management. Many farms have neither the time nor the resources (or in the case of large industrial operations the long-term financial foresight on the part of management) to engage in multi-year regenerative practices necessary to reestablish the soil mycostructure and probiotic ecosystems necessary to recover productivity.
However, if demand were to rise sufficiently and if production instead of distribution was the driving force behind food scarcity, we do indeed have arable land reserves which do not require further destruction of uncultivated wilderness to meet a growing demand.
There is actually an inverse relationship to land productivity at scale; the larger farms do extract more gross tonnage of product at lower costs of per-unit equipment and labor, but actually those operations significantly underproduce volumes of harvestable food if measured in 'per unit of land' instead of labor hours. Fields with multi-level, interconnected plant communities significantly overproduce in terms or total volume. This has been demonstrated in nearly every place on earth using very simple techniques that have varied names and different approaches to marketing, but are all very much the same thing: call it Fukuoka's "One Straw Revolution", Bill Mollison's "Permaculture", Historic Food Forests in Africa and the Decca in India, or in the Middle East "Oasis". In the Americas it still is "Three sisters" gardening. These techniques simply are not mechanized and delivering mass volumes of single commodity product into a global distribution network and that's where the difficulty and disconnect between global populations and available nutrition originates.
If there was a technological and market incentives to connect abundant and diverse harvests in rural communities with Bodegas, restaurants, and grocery store chains in the big cities without ten middle-men adding cost for the consumer without improving the product in the least, then half of the land abandonment issue could unwind and small farms could thrive, not by delivering bulk raw indistinguishable commodity to food manufacturing, but by delivering farm fresh produce to kitchens and fruit stalls.
If that were both low-cost for the consumer, and high earning for the farmer while having quality control and assured provenance then the conversation could center around making better use of even less total cropland and restoring wilderness to a near-native sustainable state.
But as it stands now, we're not short on crop land to human population ratio so much as short on effective distribution and incentives for responsible production.
/todays-soapbox
... this is kind of my area of peculiar skill and work. I've tried to steer multinational, multimillion Euro/dollar agriculture operations that produce significant volumes of global "organically produced" harvests in the direction of cooperating with rural and indigenous farming communities in conserving soil health to the benefit of all. That's been a mixed bag, so the forward momentum is with the local rice production in rural Yamagata. Same objectives, more cooperation and long term planning.