I've noticed that experienced crypto enthusiasts recommend not to use closed source wallets like Coinomi and Ownr. Open source ones are supposed to be more trustworthy. Why? Only a small number of hodlers can read the code in fact.
Because with closed source software (this doesn't only apply to wallets, but also your operating system for example), you can't be sure of what the software is doing.
It might be malicious, might have serious bugs/vulnerabilities or might use outdated libraries which are known to be exploitable.
With open source software on the other hand, you can check which libraries are used as well as check every new update being performed.
While you are right that the majority isn't able to read the code (and even if, they probably wouldn't), a few technically versed people did look at it. Some bugs or malicious backdoors are found way faster in open source software than in closed source.
If there was a malicious backdoor in electrum/wasabi/core, you can be sure that you would have already heard about it. This does not apply to closed source wallets.