You must be wondering why this is so.
First, consent can always be withdrawn. In that case, if consent was the legal basis for processing, and you do not have that consent anymore (because the person whose data are processed in this case has the right to withdraw consent at any time), you do not have the right to continue collecting data.
Second, the advisory body that dealt with the interpretation of GDPR1 even characterized the employee consent as being deceptive for the employee, and stressed that the consent was a completely inappropriate basis for data processing when it comes to employees.
When you think about it, such an explanation makes perfect sense. The employer has a certain authority and the employee is still in a subordinate position to the employer, and such employee consent could not be considered to be freely given. For example, it can be expected that the employee, as a party in a subordinate position, due to the fear of losing the job if denying the consent, might sign a consent to the processing of certain categories of data, that he would not otherwise share with a third party.
Third, consent is required only on exceptional occasions – for example, you want your employees to take their pictures for marketing purposes (for example, for printing publications, uploading photos to the website, etc.). In this case, consent is required, as this is not necessary for the employment relationship itself and, therefore, cannot be classified as another legal basis.
Fourth, when consent is given, it cannot be given in general for all processing, but for precise and precisely defined processing. At the beginning of the work relationship, not all types of processing can be foreseen, and it is not possible to give consent in advance.