The default format for wireshark/tcpdump/tshark is not a text file format. The ".cap" pcap format is a binary one. Splunk won't have the intimate knowledge of the pcap binary format to be able to process it natively.
From the manpage for tshark (command line wireshark):
If you want to write the decoded form of packets to a file,
run TShark without the -w option, and redirect its standard
output to the file (do not use the -w option).
When writing packets to a file, TShark, by default, writes the file
in libpcap format, and writes all of the packets it sees to the output file.
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