Most exchange platforms operate on deposits and withdrawals with amounts greater than $ 100, so adopting a LN wallet will be more expensive than an investment return because they will get small sums for using that service.
Many sites are taking it as evidence that the software is updated and work is underway to develop and keep the problem in the small payments and not being suitable for the platforms.
I think the real issue here is that most exchanges that transact on Lightning Network for deposits and withdrawals don't advertise it loudly enough. Otherwise, everyone like what's fast and cheap when it comes to crytp transactions