Maybe they wanted to offer all those low income folks & FWs without credit cards the ability to have a postpaid plan...
Either way, they want to operate a postpaid plan like a prepaid plan (just like M1 did with Maxx, Zero1 did with Jumbo & now Singtel is doing with Heya. Just because it is different to what you think a postpaid plan should be like, does not make it wrong.
I'll argue that's a completely different thing again. Since all these postpaid plans do not have the concept of a 'wallet', you are not paying into the number's wallet, but into your account with the telco/MVNO.
Bottomline, I don't disagree it's a bit of a clunky system. But, clearly SIMBA did this to streamline their operations. They are not a bank. I'm sure it's possible for them to create some system for you to top up odd amounts into but that comes with an upfront cost (to build the system) & a recurring cost (to maintain the system). & do their partners/resellers even want to handle such a system (manage top-ups of 1c because someone accidentally used 1.01GB of roaming data)... All these add up to costs & lower margins in a paper-thin margin game. Or just use the current system, which is also familiar & not different to how prepaid cards have worked for decades in Singapore.
& yes, if you don't like it, don't use it...