Lake Volta is a reservoir, filled by the damming of the Volta river in the early 1960s; although planned in the late colonial period, largely in an attempt to develop a source of hydro-electric power for aluminium smelting, the actual construction was part of Nkrumah's efforts to industrialise and modernise Ghana post-independence. As such, the answer to your query is that the lake was sited so as not to cause disruption to Ghana's major cities. Haphazard and unregulated construction of new settlements around the lake was deliberately restricted as part of a multi-agency government effort designed to make the whole project a symbol of the new, independent Ghana.

Construction of a small new town was authorised at Akosombo at the south end of the lake; this produced what Jackson terms a "carefully manicured" settlement, intended to house workers labouring on government dam and lake projects. Akosombo was given street names that recognised the independent nations of Africa and commemorated the independence period. The town was designed to be a showpiece, with a regulated market, good sanitation, and superior healthcare facilities. But private construction in the area was strictly controlled.

There was of course an indigenous population in the area prior to the construction of the dam, mainly comprising around 2,000 people who made a living from fishing the river, plus their families. Some of these were allowed to settle on the shores on the new lake and continued their old lives, while others accepted government offers of relocation.

Sources

Gold Coast Department of Information Services, The Volta River Project: What It Means For You (1955)

Iain Jackson et al, "The Volta River Project: planning housing and resettlement in Ghana, 1950-1965," Journal of Architecture 24 (2019)

Kwaku Obosu-Mensah, Ghana's Volta Resettlement Scheme (1996)