The only answer is because scriptures have ordained this. Sastram has laid down specifications for kainkaryams like pradakshinam (circumambulation), anjali, pranamam etc. for Perumal.
Sastram had instructed the number of times a pradakshinam can be done and how to do pradakshinam and anjali. Anjali must not be done with one hand and pradakshinam should not be done running.
Similarly, pranamam should be done in even numbers “pradakshinaan pramanaanscha yugmaanave samaacaret”; and it has listed the reason for it as well.
Everything must be in even numbers for Perumal and an odd number is not considered to be sacred.
As per this Vedic quote ‘ekAmanvaH ekahipitrunAm, druranvAH druhidevanaM’ anything done once are for Pitrus and two times are for Devas.
As Perumal is the Supreme Lord, any kainkaryams that are done should be more than just two and it could be in the count of four or six and so forth. Swami Desikan had provided many techniques on this.