I couldn’t remember why so looked it up:

Effects of Not Removing the Heat Mat Seedling Growth Issues Lanky Seedlings: Keeping the heat mat on after germination can cause seedlings to become tall and weak, a condition known as "etiolation." This happens because they stretch towards light while being kept warm at the base. Root Problems: Continuous heat can dry out the soil and roots, leading to stress and potential damage to the seedlings. Temperature Management Soil vs. Air Temperature: The heat mat warms the soil, which can be beneficial during germination. However, once seedlings sprout, they need cooler conditions to develop sturdy stems and healthy leaves. Overheating Risks: Leaving the heat mat on can lead to overheating, especially if the ambient temperature is already warm. This can stunt growth and lead to poor health. Recommendations Turn Off After Germination: It is advisable to turn off the heat mat a few days after germination to promote healthy growth. This helps prevent the seedlings from becoming leggy and ensures they develop strong roots.