If a decent percentage of homeless people are suffering from mental illness or addiction, throwing money at them won’t actually solve the problem.
For the drug addicts it could kill them, as they’ll get a windfall to go on a bender. For those with various mental illnesses, throwing money at them won’t magically give them the skills to be a good steward of the money and do everything need to get and keep a home.
There are also a non-trivial number of people who have jobs and aren’t homeless today, may end up homeless if they lose their job, even with UBI. For many, a job is their purpose and gives structure to their life. Take that away and addiction often fills the void.
UBI will help some, but not all. There will also likely be some unintended consequences in the form of addiction and death.
UBI also doesn’t mean people are living well, it’s the bare minimum, or maybe less than that. If everyone is getting the same amount, that life will not be a good one, as resources will be spread thin and corruption will creep into ranks of those who collect and distribute the money.
Someone who is homeless, without addiction or mental illness has nowhere to go but up, so sure, they’d like UBI. Distributions to that group would be the most difficult, so it’s questionable if they’d even see it.