Singapore Airlines — seat allocation

I travelled from Singapore to Heathrow, 9.10 am from Singapore on the 22nd of November. The cabin I was in was not full. I was seated in row 59 along with my husband and another man who sat on the aisle. I noticed that there were other empty seats, I saw a number of single people enjoying a four seat space in the same cabin and the cabin ahead. Why should some people enjoy that amount of space when I had to endure the entire journey sitting upright? These people were able to stretch out and have a nice rest. I cannot understand why the cabin staff, who could clearly see there were many empty spaces, did not attempt to allocate or offer space on an equal basis. I asked one of the cabin crew, a male, who said there were no other spaces available and that people had just moved seats, he said he would report it to his supervisor, he may well have done it but it made no difference. I would suggest that in this situation the cabin staff could be more vigilant about free space and distribute it fairly.