There are two very good reasons for this.
Most obviously people don't want to pay the ridiculous costs of these tolls ($5.27 or something for ~3km for the section I refuse to use).
This aside though, the even bigger problem for me is that complete and utter clusterfuck that is the toll payment system.
Time and time again me, my brother and my SO (that I know of) get letters show up demanding over $100 per toll for failure to pay. My SO has a tag with credit on it, my motorbike is linked to that tag and I'm not sure about my brother.
Even if there were insufficient funds... how convenient that the first notice for $27 or whatever it is doesn't show up, only the escalated ones?
Better yet for me, I'm still paying off ~$3k of toll fines that were wracked up in my car when I didn't even have a licence. The first I heard of it was when SPER suspended my licence until I set up a payment plan. I was told I had 14 days to resolve it (turns out thats bs) and it took about 14 days just to make my way up the ladder of dipshits at GoVia because they just don't care.
The end result is that I have been stuck paying this off for years now and I've just found out that I actually could have submitted an application to send it back to GoVia to continue the bullshit battle. To do that now though, since SPER have declined my initial request for a copy of the recorded conversation or a transcript, it looks like I'll likely have to go to magistrates court to fight for my right to my own information... so I can go to magistrates court and challenge GoVia on the matter of having to pay fines that aren't mine.
Basically, the whole system just says "fuck you, you'll pay what we say unless you've got endless time to deal with all of us".
Last time I spoke to GoVia she kept repeating the same line (essentially, pay all fines right now or go away) and when I tried to figure out exactly what my rights and options were, she asked if I was making payment right now then hung up on me.
TL;DR: FUCK GoVia, I now prefer to run the risk of being late for meetings/appointments rather than take the tunnels. If GoVia ceased to exist, I would consider using them but even then only on the motorbike.