Basically for two reasons:

1. Despite the land-ownership and agricultural aspirations of Zionists, most of the Jews of Palestine were living in cities or towns in 1948. That included 150,000 in Tel Aviv alone in 1940.

2. 7% of the total land of Palestine doesn't necessarily tell you much about the usefulness of that land (edit: rather, about the usefulness of the remaining 93%). Although the Zionists were keen to "make the desert bloom" and often enjoyed great success in making previously poor land more productive, about half of the land was and remains totally unsuitable to agriculture and is barely habitable. This becomes clear on looking at a satellite view of the country; the entire southern half of Palestine is dominated by the Negev desert: https://www.google.com/maps/place/Israel/@31.1012063,33.9738081,496705m/data=!3m1!1e3!4m5!3m4!1s0x1500492432a7c98b:0x6a6b422013352cba!8m2!3d31.046051!4d34.851612

While the other answer mentions some practicality restrictions, and city-dwelling, the other answer misses out on another crucial factor, which was that no one wanted to sell land to Jews.

Even before the 1939 White Paper limited the areas where Jews could buy land, the stigma of selling land to Jews became a very potent one. This stemmed from the sense that Jews were "usurping" the land, and also from a policy that some Jewish organizations had of using Jewish labor almost exclusively. Whereas previously, land had changed hands many times without the families/peasants living on it having to change anything besides who they paid some portion of their incomes to, now Jewish landowners were also removing those families and beginning to employ other Jews. This was intended to show that Jews were both capable and ready to be a self-sustaining economic force, capable of running a state efficiently, but it had the side-effect of some displacement of Arab peasants, though the magnitude is disputed.

This stigma meant that selling land to Jews was considered a serious act of, if not treason to some extent on a national scale, then of betrayal to those who lived on the land.

Most land, before the 1920s in particular, was bought in sparsely populated areas, which helped avoid this issue (though it was still brought up). Following that period, Jews began to purchase more heavily from non-Palestinian owners, primarily those living outside of the area, who were more willing to sell and had less fear of repercussions. These landowners held plots in more populated areas, which meant displacement became a growing issue. The British Peel Commission Report of 1937, which proposed a two-state solution for the first time in the form of partition (accepted in principle by the Jews but rejected in substance, and rejected in total by the Arabs), wrote of this thusly:

In 1931, Arabs claiming they had been displaced from lands owing to Jewish purchase thereof were invited to submit their claims for registration with a view to resettlement at Government cost. To guard against further "landlessness", a Protection of Cultivators Ordinance had been enacted in 1929, securing tenants from arbitrary removal from land in the event of its sale. After several amendments, this was replaced by a new Ordinance in 1933. In 1931, and again in 1933, Mr. C. F. Strickland visited the country to promote, by inquiry and report and by personal propaganda throughout the country, the growth of cooperative societys--a measure of assistance again, which though helpful to both races, was needed less by the Jews, who had long established an extensive and varied system of cooperation, than by the Arabs, who knew virtually nothing about it.

The White Paper of 1939, meant to help mollify Arab anger over this and other issues like Jewish immigration and the idea of a Jewish state, placed limits on Jewish immigration, placed a 10 year time period before the potential establishment of a single, unitary state, and limited land purchase by Jews as well. The White Paper, as the UN Partition Plan of 1947 notes, said that:

the country was divided into three zones: in the first two (covering 95 per cent of the area of Palestine) the transfer of land to Jews by Palestinian Arabs was either prohibited or required the sanction of the Government; in the third or "free" zone an area of some 332,000 acres (5 per cent of the total area of Palestine), mainly in the coastal plain but including land around Jerusalem, all land purchased was unrestricted.

These legal restrictions were less effective, as I mentioned, than the overall stigma at any rate. It got so severe that in 1934, Jerusalem's Mufti Hajj Amin al Husseini decreed that Muslims who sold land to Jews must be murdered by believers, but murder of the sort had been a fixture of the area even before then. Indeed, the Husseinis had begun to encourage such murder in the 1920s, and emblematic of this trend was the murder of Sheikh Musa Hadeib. It is widely believed, though it was never confirmed, that he was killed by followers of the Husseinis, for "collaboration", because he appeared to be in favor of the British Mandate and also because he may have sold lands to Jews. Land sales were also part of the impetuses for the 1920, 1921, and 1929 Arab riots in the area (along with the aforementioned immigration disputes, for example).

To get around the stigma and the laws restricting Jewish land purchase, Jews typically had to pay a third party to purchase the land, making it far more expensive. This method meant that Jewish land ownership was not just stigmatized and not just made almost impossible, it was also prohibitively expensive for many.

So while there wasn't always a need for such land purchase, it certainly would've been higher had there not been that stigma and policy.