Because it is not illegal to let your player play on field different from his specialization (1 to 3 fields out of all 14 fields). When your player plays on fields his is not specialized to, his performance his slightly worse, but if his skills are huge he still could play better than opponents player on right field. In the end you lose some ball possession by player on wrong field, but it could be from 1 to 5 % (no one knows because Nordeus has not revealed game equation yet nor tells us exactly the penalization for playing players on wrong fields).
So, having GK on ST field (instead of ST) is better than have no ST, because formation without any ST is illegal. But no one knows if it is better to have on ST field brilliant GK with high attacking skills or ST with skills under average. And no one knows what is the skill compensation for playing wrong field (let us say e.g. that playing on ST without specialization to ST reduces all skills of such a player by 5 points during such a match). I think Nordeus owes us such an explanation.