Why people don't like Lulu being viable at lanes other than support?

I sincerely don't understand that. Lulu is on a pretty good spot right now and does not need any changes. She is already a pretty good support, I use her when I have to go support, as a counter to engage teams, assassins, and if I need someone that can protect teammates like no one else. That said, I don't understand why people don't like her being used at top and mid.

Lulu is viable and strong, can bully many melees, is one of the few mages that is not weak to assassins, but taking her at mid and top have its downsides.

For example, if you are taking Lulu top, you are sacrificing a spot that could be used for a tanky champion, for a squishy champion that has some damage potential and a lot of utility and CC. She is still squishy, however, and can't make up for that. Unless you compensate that somewhere, such as on the jungle, your team will be very squishy and likely to lose all teamfights due to having no one that can absorb the initial burst and protect the key carries.

And if you are taking her mid, you are losing out a lot of damage potential. Lulu's damage is lower than the average mage, since she has only two damaging abilities (not including her passive) and those two abilities have poor AP ratios. If you are using Lulu mid, it is for her CC, utility, and ability to counter a lot of metagame threats, including Cho'Gath and some assassins. If you were to take someone with damage, it would be better to just use another mage.

And lastly, she has issues dealing with tanks. She has no % abilities, has poor ratios, and has only two damaging abilities. She will have a very hard time dealing relevant damage to tanks. This is relevant on that meta because tanks are popular. She has her own ways to deal with tanks, but damage is not one of them, she needs to rely on her teammates to be able to properly deal with tanks.

However, her utility is so amazing that she can make up for not having as much damage as other champions. She is very good when you have a hypercarry on your team, such as Kog'Maw, since then you will essentially have two supports ready to protect him (three if you count the jungler which will likely be a tank). She also needs to stay viable at mid and top because, as I said above, she is one of the few counters to assassins, bruisers, and some burst mages alike.

I don't know why some people want Riot to change her to be exclusively used as a support. You want diversity, yet you don't want Lulu to have diversity on what she can do? She has already been nerfed, she is balanced already. Lulu is strong but she isn't OP or broken, like other champions, such as LeBlanc before the 5.11 changes.