In our textbook it says that translation occurs more in a cell than transcription but I couldn't find anything that explains why it happens
The simple answer
Under the assumption that each mRNA molecule is translated at least once, by necessity translation will happen more often than transcription. This is because the only way to get a protein is to translate an mRNA.
In other words, as long as there are more protein molecules (translation products) than mRNA molecules in a cell, then the process of translation must have occurred more than transcription.
One estimate of the ratio of protein molecules to mRNA molecules given here is on the order of 100-1000 for E. coli. ("Under exponential growth at medium growth rate E. coli is known to contain about 3 million proteins and 3000 mRNA (BNID 100088, 100064). These constants imply that the protein to mRNA ratio is ≈1000, in line with the estimate given above. [MP note: the numbers quoted are actually closer to ~400K and ~1.4K, so this is not completely accurate; the source paper itself estimates between $10^2-10^4$ for the same ratio of protein to mRNA. Note though that the yeast cell estimates are even higher.]) So there are likely between 100 and 10,000 times as many translation as transcription events.
The speculative answer
From the perspective of "why" this is the case in terms of why cells might "choose" to have this ratio of proteins to mRNA (accepting that it must be true based on the ratio of protein to mRNA), we might say that proteins are useful and mRNA is not. Therefore, it's better to have more proteins (implied but not stated by @mgkrebbs comment). Or rather, mRNA is only useful because it gets you proteins, which are useful.
So it "makes sense" for cells to minimize the amount of energy put into generating mRNA and maximize energy put into making proteins. But that's human logic projected onto the cell, we can't know that that is the logic for why cells do what they do (or evolved to do, or whatever).
It would be very wasteful (and hence evolutionary unfavorable) for cells to have the process other way round (more mRNA than protein). Remember it takes a lot of energy to synthesize mRNA, so it must be used as efficiently as possible (make many protein copies from a single mRNA).