The domains of input vary greatly. Even in exotic cases, mathematical inputs are very rigorously defined so there is abundant clarity on what the inputs are. With computing the domain of inputs is so vast: a computer program can accept humble numbers (the simplest to verify) or touch input from a smart phone. The ever expanding appetite for digitisation, and by extension computability, implies continuously increasing complexity of software beyond the bounds of what can easily be verified. This need for digitisation strains our current abilities to 'keep up' leading to what we commonly refer to as 'bugs'.
Here's a quote from https://www.technologyreview.com/2002/07/01/40875/why-software-is-so-bad/ which illustrates this:
“The classic dilemma in software is that people continually want more and more and more stuff,” says Nathan Myhrvold, former chief technology officer of Microsoft. Unfortunately, he notes, the constant demand for novelty means that software is always “in the bleeding-edge phase,” when products are inherently less reliable. In 1983, he says, Microsoft Word had only 27,000 lines of code. “Trouble is, it didn’t do very much”-which customers today wouldn’t accept. If Microsoft had not kept pumping up Word with new features, the product would no longer exist.