Lambs are usually slaughtered after 6 months. In Sweden, to use an example close to me, lambs have the right to live outdoor for two to four months during the summer depending on where in the country they live, which means that they spend at least 2-4 months indoors not grazing this non-arable land. They must be watered and fed, which increases energy consumption, and usually fed from land that could have grown crops for humans directly. Only a fraction of the energy we put into animals gets returned when we eat them, which means it's far more energy-efficient to eat the plants directly.